[llvm-dwp] Join dwo paths correctly when DWOPath is absolute

When the `DWOPath` is absolute, we want to use `DWOPath` as is, without prepending any other
components to the path. The `sys::path::append` does not join, but rather unconditionally appends
the paths, so something like `sys::path::append("/tmp", "/tmp/banana")` will result in
`/tmp/tmp/banana` rather than the desired `/tmp/banana`.

This then causes `llvm-dwp` to fail in a following situation:

```
$ clang -gsplit-dwarf /tmp/banana/test.c -c -o /tmp/outdir/foo.o
$ clang outdir/foo.o -o outdir/hm
$ llvm-dwarfdump outdir/hm | grep -C2 foo.dwo
                  DW_AT_comp_dir    ("/tmp")
                  DW_AT_GNU_pubnames  (true)
                  DW_AT_GNU_dwo_name    ("/tmp/outdir/foo.dwo")
                                DW_AT_GNU_dwo_id    (0xde4d396f3bf0e257)
                  DW_AT_low_pc  (0x0000000000401100)
$ strace -o trace llvm-dwp -e outdir/hm -o outdir/hm.dwp
error: No such file or directory
$ cat trace | grep foo.dwo
openat(AT_FDCWD, "/tmp/tmp/outdir/foo.dwo", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
```

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D96678

Backport of: 6ffcb29

llvm/test/tools/llvm-dwp/X86/absolute_paths.test

@@ -0,0 +1,37 @@
+; RUN: rm -rf %t
+; RUN: mkdir -p %t
+; RUN: llc %s -mtriple=x86_64-linux --split-dwarf-file=%t/test.dwo --split-dwarf-output=%t/test.dwo --filetype=obj -o %t/test.o
+; RUN: llvm-dwarfdump -v %t/test.dwo | FileCheck %s -DPATH=%t
+; RUN: llvm-dwp -e %t/test.o -o %t/test.dwp
+; RUN: llvm-dwarfdump -v %t/test.dwp | FileCheck %s -DPATH=%t
+
+; CHECK-LABEL: .debug_abbrev.dwo contents:
+; CHECK: DW_AT_name
+; CHECK: DW_AT_GNU_dwo_name
+; CHECK: DW_AT_name
+; CHECK-LABEL: .debug_str.dwo contents:
+; CHECK: "banana"
+; CHECK: "/tmp/test.c"
+; CHECK: "[[PATH]]/test.dwo"
+
+define void @banana() !dbg !8 {
+  ret void, !dbg !12
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.1", isOptimized: true, runtimeVersion: 0, splitDebugFilename: "test.dwo", emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: GNU)
+!1 = !DIFile(filename: "/tmp/test.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 11.0.1"}
+!8 = distinct !DISubprogram(name: "banana", scope: !9, file: !9, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!9 = !DIFile(filename: "test.c", directory: "/tmp")
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !DILocation(line: 1, column: 20, scope: !8)

llvm/tools/llvm-dwp/llvm-dwp.cpp

@@ -526,8 +526,8 @@ getDWOFilenames(StringRef ExecFilename) {
     std::string DWOCompDir =
         dwarf::toString(Die.find(dwarf::DW_AT_comp_dir), "");
     if (!DWOCompDir.empty()) {
-      SmallString<16> DWOPath;
-      sys::path::append(DWOPath, DWOCompDir, DWOName);
+      SmallString<16> DWOPath(std::move(DWOName));
+      sys::fs::make_absolute(DWOCompDir, DWOPath);
       DWOPaths.emplace_back(DWOPath.data(), DWOPath.size());
     } else {
       DWOPaths.push_back(std::move(DWOName));