clang/Lex: Stop using SourceManager::getBuffer

[dexonsmith]

Summary

Update clang/lib/Lex to stop relying on a MemoryBuffer*, using the
MemoryBufferRef from getBufferOrNone since both locations had logic
for checking validity of the buffer. There's potentially a functionality
change, since the logic was wrong (it checked for nullptr, which was
never returned by the old API), but if that was reachable the new
behaviour should be better.

clang/lib/Lex/ModuleMap.cpp

bool ModuleMap::parseModuleMapFile(const FileEntry *File, bool IsSystem,
   // If the module map file wasn't already entered, do so now.
   if (ID.isInvalid()) {
     auto FileCharacter =
         IsSystem ? SrcMgr::C_System_ModuleMap : SrcMgr::C_User_ModuleMap;
     ID = SourceMgr.createFileID(File, ExternModuleLoc, FileCharacter);
   }
 
   assert(Target && "Missing target information");
-  const llvm::MemoryBuffer *Buffer = SourceMgr.getBuffer(ID);
+  llvm::Optional<llvm::MemoryBufferRef> Buffer = SourceMgr.getBufferOrNone(ID);
   if (!Buffer)
     return ParsedModuleMap[File] = true;
   assert((!Offset || *Offset <= Buffer->getBufferSize()) &&
          "invalid buffer offset");
 
   // Parse this module map file.
   Lexer L(SourceMgr.getLocForStartOfFile(ID), MMapLangOpts,
           Buffer->getBufferStart(),

clang/lib/Lex/PPDirectives.cpp

Optional<unsigned> Preprocessor::getSkippedRangeForExcludedConditionalBlock(
     SourceLocation HashLoc) {
   if (!ExcludedConditionalDirectiveSkipMappings)
     return None;
   if (!HashLoc.isFileID())
     return None;
 
   std::pair<FileID, unsigned> HashFileOffset =
       SourceMgr.getDecomposedLoc(HashLoc);
-  const llvm::MemoryBuffer *Buf = SourceMgr.getBuffer(HashFileOffset.first);
+  Optional<llvm::MemoryBufferRef> Buf =
+      SourceMgr.getBufferOrNone(HashFileOffset.first);
   if (!Buf)
     return None;
   auto It =
       ExcludedConditionalDirectiveSkipMappings->find(Buf->getBufferStart());
   if (It == ExcludedConditionalDirectiveSkipMappings->end())
     return None;
 
   const PreprocessorSkippedRangeMapping &SkippedRanges = *It->getSecond();