[Support] Use final filename for Caching buffer identifier

[lgrey]

Summary

Mach-O LLD uses the buffer identifier of the memory buffer backing an object file to generate stabs which are used by dsymutil to find the object file for dSYM generation.

When using thinLTO, these buffers are provided by the cache which initially saves them to disk as temporary files beginning with "Thin-" but renames them to persistent files beginning with "llvmcache-" before the buffer is provided to the cache user.

However, the buffer is created before the file is renamed and is given the temp file's name as an identifier. This causes the generated stabs to point to nonexistent files.

This change names the buffer with the eventual persistent filename. I think this is safe because failing to rename the temp file is a fatal error.

[thakis]

Is it possible to test this?

[thakis]

A manual test looks like this:

% out/gn/bin/clang -c -flto=thin main.cc -g
% out/gn/bin/clang -flto=thin main.o -fuse-ld=lld -Wl,-cache_path_lto,thinltocache
% dsymutil a.out

Without this change, this prints:

% dsymutil a.out
warning: (x86_64) /Users/thakis/src/llvm-project/thinltocache/Thin-6c0491.tmp.o unable to open object file: No such file or directory
warning: no debug symbols in executable (-arch x86_64)

With this change, it doesn't print that.

[thakis]

lg. I went ahead and committed it, with a test added. Please take a look at the test and check that it's along the lines you imagined.

lld/test/MachO/lto-cache-dsymutil.ll

+; REQUIRES: x86
+
+; RUN: rm -rf %t; mkdir %t
+; RUN: opt -module-hash -module-summary %s -o %t/foo.o
+; RUN: %lld -cache_path_lto %t/cache -o %t/test %t/foo.o
+
+;; Check that dsymutil is able to read the .o file out of the thinlto cache.
+; RUN: dsymutil -f -o - %t/test | llvm-dwarfdump - | FileCheck %s
+; CHECK: DW_AT_name ("test.cpp")
+
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @main() #0 !dbg !4 {
+  ret void
+}
+
+!llvm.module.flags = !{ !0, !1 }
+!llvm.dbg.cu = !{!2}
+
+!0 = !{i32 7, !"Dwarf Version", i32 4}
+!1 = !{i32 2, !"Debug Info Version", i32 3}
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, emissionKind: FullDebug)
+!3 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!4 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 1, type: !5, scopeLine: 1, unit: !2)
+!5 = !DISubroutineType(types: !6)
+!6 = !{}

llvm/lib/Support/Caching.cpp

struct CacheStream : CachedFileStream {
         // allowing to replace report_fatal_errors with a return Error.
 
         // Make sure the stream is closed before committing it.
         OS.reset();
 
         // Open the file first to avoid racing with a cache pruner.
         ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
             MemoryBuffer::getOpenFile(
-                sys::fs::convertFDToNativeFile(TempFile.FD), TempFile.TmpName,
+                sys::fs::convertFDToNativeFile(TempFile.FD), EntryPath,
                 /*FileSize=*/-1, /*RequiresNullTerminator=*/false);
         if (!MBOrErr)
           report_fatal_error(Twine("Failed to open new cache file ") +
                              TempFile.TmpName + ": " +
                              MBOrErr.getError().message() + "\n");
 
         // On POSIX systems, this will atomically replace the destination if
         // it already exists. We try to emulate this on Windows, but this may