Upgrade to LLVM's master branch (LLVM 7)

This commit upgrades the main LLVM submodule to LLVM's current master branch.
The LLD submodule is updated in tandem as well as compiler-builtins.

Along the way support was also added for LLVM 7's new features. This primarily
includes the support for custom section concatenation natively in LLD so we now
add wasm custom sections in LLVM IR rather than having custom support in rustc
itself for doing so.

Some other miscellaneous changes are:

* We now pass `--gc-sections` to `wasm-ld`
* The optimization level is now passed to `wasm-ld`
* A `--stack-first` option is passed to LLD to have stack overflow always cause
  a trap instead of corrupting static data
* The wasm target for LLVM switched to `wasm32-unknown-unknown`.
* The syntax for aligned pointers has changed in LLVM IR and tests are updated
  to reflect this.
* The `thumbv6m-none-eabi` target is disabled due to an [LLVM bug][llbug]

Nowadays we've been mostly only upgrading whenever there's a major release of
LLVM but enough changes have been happening on the wasm target that there's been
growing motivation for quite some time now to upgrade out version of LLD. To
upgrade LLD, however, we need to upgrade LLVM to avoid needing to build yet
another version of LLVM on the builders.

The revision of LLVM in use here is arbitrarily chosen. We will likely need to
continue to update it over time if and when we discover bugs. Once LLVM 7 is
fully released we can switch to that channel as well.

[llbug]: https://bugs.llvm.org/show_bug.cgi?id=37382

Author: alexcrichton

src/libcompiler_builtins

@@ -665,8 +665,6 @@ define_dep_nodes!( <'tcx>
 
     [] InstanceDefSizeEstimate { instance_def: InstanceDef<'tcx> },
 
-    [] WasmCustomSections(CrateNum),
-
     [input] Features,
 
     [] ProgramClausesFor(DefId),

src/librustc/dep_graph/dep_node.rs

@@ -57,7 +57,7 @@ struct CheckAttrVisitor<'a, 'tcx: 'a> {
 impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
     /// Check any attribute.
     fn check_attributes(&self, item: &hir::Item, target: Target) {
-        if target == Target::Fn {
+        if target == Target::Fn || target == Target::Const {
             self.tcx.codegen_fn_attrs(self.tcx.hir.local_def_id(item.id));
         } else if let Some(a) = item.attrs.iter().find(|a| a.check_name("target_feature")) {
             self.tcx.sess.struct_span_err(a.span, "attribute should be applied to a function")
@@ -85,11 +85,6 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
                 if target != Target::Const {
                     self.tcx.sess.span_err(attr.span, "only allowed on consts");
                 }
-
-                if attr.value_str().is_none() {
-                    self.tcx.sess.span_err(attr.span, "must be of the form \
-                        #[wasm_custom_section = \"foo\"]");
-                }
             }
         }
 

src/librustc/hir/check_attr.rs

@@ -2259,6 +2259,7 @@ pub struct CodegenFnAttrs {
     pub export_name: Option<Symbol>,
     pub target_features: Vec<Symbol>,
     pub linkage: Option<Linkage>,
+    pub wasm_custom_section: Option<Symbol>,
 }
 
 bitflags! {
@@ -2283,6 +2284,7 @@ impl CodegenFnAttrs {
             export_name: None,
             target_features: vec![],
             linkage: None,
+            wasm_custom_section: None,
         }
     }
 

src/librustc/hir/mod.rs

@@ -1120,6 +1120,7 @@ impl_stable_hash_for!(struct hir::CodegenFnAttrs {
     export_name,
     target_features,
     linkage,
+    wasm_custom_section,
 });
 
 impl<'hir> HashStable<StableHashingContext<'hir>> for hir::CodegenFnAttrFlags

src/librustc/ich/impls_hir.rs

@@ -24,6 +24,7 @@ pub enum MonoItem<'tcx> {
     Fn(Instance<'tcx>),
     Static(DefId),
     GlobalAsm(NodeId),
+    CustomSection(DefId),
 }
 
 impl<'tcx> MonoItem<'tcx> {
@@ -36,7 +37,9 @@ impl<'tcx> MonoItem<'tcx> {
             },
             // Conservatively estimate the size of a static declaration
             // or assembly to be 1.
-            MonoItem::Static(_) | MonoItem::GlobalAsm(_) => 1,
+            MonoItem::Static(_) |
+            MonoItem::GlobalAsm(_) |
+            MonoItem::CustomSection(_) => 1,
         }
     }
 }
@@ -51,7 +54,8 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for MonoItem<'tcx> {
             MonoItem::Fn(ref instance) => {
                 instance.hash_stable(hcx, hasher);
             }
-            MonoItem::Static(def_id) => {
+            MonoItem::Static(def_id) |
+            MonoItem::CustomSection(def_id) => {
                 def_id.hash_stable(hcx, hasher);
             }
             MonoItem::GlobalAsm(node_id) => {

src/librustc/mir/mono.rs

@@ -776,12 +776,6 @@ impl<'tcx> QueryDescription<'tcx> for queries::instance_def_size_estimate<'tcx>
     }
 }
 
-impl<'tcx> QueryDescription<'tcx> for queries::wasm_custom_sections<'tcx> {
-    fn describe(_tcx: TyCtxt, _: CrateNum) -> String {
-        format!("custom wasm sections for a crate")
-    }
-}
-
 impl<'tcx> QueryDescription<'tcx> for queries::generics_of<'tcx> {
     #[inline]
     fn cache_on_disk(def_id: Self::Key) -> bool {

src/librustc/ty/query/config.rs

@@ -546,7 +546,6 @@ define_queries! { <'tcx>
         ty::ParamEnv<'tcx>
     ) -> Clauses<'tcx>,
 
-    [] fn wasm_custom_sections: WasmCustomSections(CrateNum) -> Lrc<Vec<DefId>>,
     [] fn wasm_import_module_map: WasmImportModuleMap(CrateNum)
         -> Lrc<FxHashMap<DefId, String>>,
 }

src/librustc/ty/query/mod.rs

@@ -1208,7 +1208,6 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::Features => { force!(features_query, LOCAL_CRATE); }
 
         DepKind::ProgramClausesFor => { force!(program_clauses_for, def_id!()); }
-        DepKind::WasmCustomSections => { force!(wasm_custom_sections, krate!()); }
         DepKind::WasmImportModuleMap => { force!(wasm_import_module_map, krate!()); }
         DepKind::ForeignModules => { force!(foreign_modules, krate!()); }
 

src/librustc/ty/query/plumbing.rs

@@ -11,9 +11,8 @@
 
 use std::ffi::{CStr, CString};
 
-use rustc::hir::{self, CodegenFnAttrFlags};
+use rustc::hir::CodegenFnAttrFlags;
 use rustc::hir::def_id::{DefId, LOCAL_CRATE};
-use rustc::hir::itemlikevisit::ItemLikeVisitor;
 use rustc::session::Session;
 use rustc::session::config::Sanitizer;
 use rustc::ty::TyCtxt;
@@ -222,37 +221,9 @@ pub fn provide(providers: &mut Providers) {
         }
     };
 
-    providers.wasm_custom_sections = |tcx, cnum| {
-        assert_eq!(cnum, LOCAL_CRATE);
-        let mut finder = WasmSectionFinder { tcx, list: Vec::new() };
-        tcx.hir.krate().visit_all_item_likes(&mut finder);
-        Lrc::new(finder.list)
-    };
-
     provide_extern(providers);
 }
 
-struct WasmSectionFinder<'a, 'tcx: 'a> {
-    tcx: TyCtxt<'a, 'tcx, 'tcx>,
-    list: Vec<DefId>,
-}
-
-impl<'a, 'tcx: 'a> ItemLikeVisitor<'tcx> for WasmSectionFinder<'a, 'tcx> {
-    fn visit_item(&mut self, i: &'tcx hir::Item) {
-        match i.node {
-            hir::ItemConst(..) => {}
-            _ => return,
-        }
-        if i.attrs.iter().any(|i| i.check_name("wasm_custom_section")) {
-            self.list.push(self.tcx.hir.local_def_id(i.id));
-        }
-    }
-
-    fn visit_trait_item(&mut self, _: &'tcx hir::TraitItem) {}
-
-    fn visit_impl_item(&mut self, _: &'tcx hir::ImplItem) {}
-}
-
 pub fn provide_extern(providers: &mut Providers) {
     providers.wasm_import_module_map = |tcx, cnum| {
         let mut ret = FxHashMap();