clang/CodeGen: Stop using SourceManager::getBuffer, NFC

[dexonsmith]

Summary

Update clang/lib/CodeGen to use a MemoryBufferRef from getBufferOrNone instead of MemoryBuffer* from getBuffer. No functionality change here.

clang/lib/CodeGen/CGDebugInfo.cpp

 CGDebugInfo::computeChecksum(FileID FID, SmallString<32> &Checksum) const {
   Checksum.clear();
 
   if (!CGM.getCodeGenOpts().EmitCodeView &&
       CGM.getCodeGenOpts().DwarfVersion < 5)
     return None;
 
   SourceManager &SM = CGM.getContext().getSourceManager();
-  bool Invalid;
-  const llvm::MemoryBuffer *MemBuffer = SM.getBuffer(FID, &Invalid);
-  if (Invalid)
+  Optional<llvm::MemoryBufferRef> MemBuffer = SM.getBufferOrNone(FID);
+  if (!MemBuffer)
     return None;
 
   llvm::MD5 Hash;
   llvm::MD5::MD5Result Result;
 
   Hash.update(MemBuffer->getBuffer());
   Hash.final(Result);
 

clang/lib/CodeGen/CodeGenAction.cpp

if (getCurrentFileKind().getLanguage() == Language::LLVM_IR) {
     CompilerInstance &CI = getCompilerInstance();
     auto &CodeGenOpts = CI.getCodeGenOpts();
     auto &Diagnostics = CI.getDiagnostics();
     std::unique_ptr<raw_pwrite_stream> OS =
         GetOutputStream(CI, getCurrentFile(), BA);
     if (BA != Backend_EmitNothing && !OS)
       return;
 
-    bool Invalid;
     SourceManager &SM = CI.getSourceManager();
     FileID FID = SM.getMainFileID();
-    const llvm::MemoryBuffer *MainFile = SM.getBuffer(FID, &Invalid);
-    if (Invalid)
+    Optional<MemoryBufferRef> MainFile = SM.getBufferOrNone(FID);
+    if (!MainFile)
       return;
 
     TheModule = loadModule(*MainFile);
     if (!TheModule)
       return;
 
     const TargetOptions &TargetOpts = CI.getTargetOpts();
     if (TheModule->getTargetTriple() != TargetOpts.Triple) {
       Diagnostics.Report(SourceLocation(),
                          diag::warn_fe_override_module)
           << TargetOpts.Triple;
       TheModule->setTargetTriple(TargetOpts.Triple);
     }
 
-    EmbedBitcode(TheModule.get(), CodeGenOpts,
-                 MainFile->getMemBufferRef());
+    EmbedBitcode(TheModule.get(), CodeGenOpts, *MainFile);
 
     LLVMContext &Ctx = TheModule->getContext();
     Ctx.setInlineAsmDiagnosticHandler(BitcodeInlineAsmDiagHandler,
                                       &Diagnostics);
 
     // Set clang diagnostic handler. To do this we need to create a fake
     // BackendConsumer.
     BackendConsumer Result(BA, CI.getDiagnostics(), CI.getHeaderSearchOpts(),