llvm: mangle extern Wasm functions

When Wasm extern functions contain the same name, but have a
different module name such as `extern "a"` vs `extern "b"` LLVM will
currently resolve the two functions to the same symbol. By mangling
the name of the symbol, we ensure the functions are resolved
seperately. We mangle the name by applying <name>|<module> where
module is also known as the library name.

src/codegen/llvm.zig

@@ -1279,8 +1279,18 @@ pub const Object = struct {
         const llvm_global = self.decl_map.get(decl_index) orelse return;
         const decl = module.declPtr(decl_index);
         if (decl.isExtern()) {
-            llvm_global.setValueName(decl.name);
-            if (self.getLlvmGlobal(decl.name)) |other_global| {
+            const is_wasm_fn = module.getTarget().isWasm() and try decl.isFunction();
+            const mangle_name = is_wasm_fn and
+                decl.getExternFn().?.lib_name != null and
+                !std.mem.eql(u8, std.mem.sliceTo(decl.getExternFn().?.lib_name.?, 0), "c");
+            const decl_name = if (mangle_name) name: {
+                const tmp = try std.fmt.allocPrintZ(module.gpa, "{s}|{s}", .{ decl.name, decl.getExternFn().?.lib_name.? });
+                break :name tmp.ptr;
+            } else decl.name;
+            defer if (mangle_name) module.gpa.free(std.mem.sliceTo(decl_name, 0));
+
+            llvm_global.setValueName(decl_name);
+            if (self.getLlvmGlobal(decl_name)) |other_global| {
                 if (other_global != llvm_global) {
                     log.debug("updateDeclExports isExtern()=true setValueName({s}) conflict", .{decl.name});
                     try self.extern_collisions.put(module.gpa, decl_index, {});