CodeGen: Use the source filename as the argument to .file, rather than the module ID.

[pcc]

Summary

Using the module ID here is wrong for a couple of reasons:

1. The module ID is not persisted, so we can end up with different object file contents given the same input file (for example if the same file is accessed via different paths).
2. With ThinLTO the module ID field may contain the path to a bitcode file, which is incorrect, as the .file argument is supposed to contain the path to a source file.

[pcc]

Ping.

[mehdi_amini]

(Sorry for the delay I didn't see this one)

LGTM.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

bool AsmPrinter::doInitialization(Module &M) {
 
   // Allow the target to emit any magic that it wants at the start of the file.
   EmitStartOfAsmFile(M);
 
   // Very minimal debug info. It is ignored if we emit actual debug info. If we
   // don't, this at least helps the user find where a global came from.
   if (MAI->hasSingleParameterDotFile()) {
     // .file "foo.c"
-    OutStreamer->EmitFileDirective(M.getModuleIdentifier());
+    OutStreamer->EmitFileDirective(M.getSourceFileName());
   }
 
   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
   assert(MI && "AsmPrinter didn't require GCModuleInfo?");
   for (auto &I : *MI)
     if (GCMetadataPrinter *MP = GetOrCreateGCPrinter(*I))
       MP->beginAssembly(M, *MI, *this);
 

llvm/test/CodeGen/X86/file-source-filename.ll

+; RUN: llc -march=x86-64 < %s | FileCheck %s
+; CHECK: .file "foobar"
+
+source_filename = "foobar"