[NFC][ThinLTO] Let llvm::EmbedBitcodeInModule handle serialization.

[mtrofin]

Summary

llvm::EmbedBitcodeInModule handles serializing the passed-in module, if
the provided MemoryBufferRef is invalid. This is already the path taken
in one of the uses of the API - clang::EmbedBitcode, when called from
BackendConsumer::HandleTranslationUnit - so might as well do the same
here and reduce (by very little) code duplication.

The only difference this patch introduces is that the serialization happens
with ShouldPreserveUseListOrder set to true.

[tejohnson]

Does this rely on the following check in EmbedBitcodeInModule returning true?

if (!isBitcode((const unsigned char *)Buf.getBufferStart(),
               (const unsigned char *)Buf.getBufferEnd())) {

I.e. I assume an empty buffer will be !isBitcode? The comments there suggest this is trying to detect whether we have bitcode vs LLVM assembly as input. Might be better to add an explicit check for an empty buffer and expand the comments (along with your current change).

[mtrofin]

comments

[mtrofin]

explicit check

[tejohnson]

LGTM with comment updates as suggested below.

llvm/include/llvm/Bitcode/BitcodeWriter.h

159 ↗

(On Diff #290763)

maybe make this something like "...if the provided Buf is not bitcode (e.g. is LLVM assembly)" since that seems to be what it is trying to handle

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

4839 ↗

(On Diff #290763)

Update comment.

[mtrofin]

comments

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

4839 ↗

(On Diff #290763)

none of that comment applies, really

llvm/lib/LTO/LTOBackend.cpp

 
 static cl::opt<bool> EmbedBitcode(
     "lto-embed-bitcode", cl::init(false),
     cl::desc("Embed LLVM bitcode in object files produced by LTO"));
 
 static void EmitBitcodeSection(Module &M) {
   if (!EmbedBitcode)
     return;
-  SmallVector<char, 0> Buffer;
-  raw_svector_ostream OS(Buffer);
-  WriteBitcodeToFile(M, OS);
-
-  std::unique_ptr<MemoryBuffer> Buf(
-      new SmallVectorMemoryBuffer(std::move(Buffer)));
-  llvm::EmbedBitcodeInModule(M, Buf->getMemBufferRef(), /*EmbedBitcode*/ true,
+  llvm::EmbedBitcodeInModule(M, llvm::MemoryBufferRef(), /*EmbedBitcode*/ true,
                              /*EmbedMarker*/ false, /*CmdArgs*/ nullptr);
 }
 
 void codegen(const Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
              unsigned Task, Module &Mod,
              const ModuleSummaryIndex &CombinedIndex) {
   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
     return;