Enable ODR uniquing of DITypes for ThinLTO backends

[tejohnson]

Summary

This is a follow-on to apply Duncan's new DIType ODR uniquing from
r266549 and r266713 in more places.

When invoking ThinLTO backend compiles via clang (for a distributed
build), invoke enableDebugTypeODRUniquing() before parsing the module.

[mehdi_amini]

LGTM
(maybe add a one line comment)

lib/CodeGen/CodeGenAction.cpp

if (getCurrentFileKind() == IK_LLVM_IR) {
 
     bool Invalid;
     SourceManager &SM = CI.getSourceManager();
     FileID FID = SM.getMainFileID();
     llvm::MemoryBuffer *MainFile = SM.getBuffer(FID, &Invalid);
     if (Invalid)
       return;
 
+    if (!CI.getCodeGenOpts().ThinLTOIndexFile.empty())
+      VMContext->enableDebugTypeODRUniquing();
+
     llvm::SMDiagnostic Err;
     TheModule = parseIR(MainFile->getMemBufferRef(), Err, *VMContext);
     if (!TheModule) {
       // Translate from the diagnostic info to the SourceManager location if
       // available.
       // TODO: Unify this with ConvertBackendLocation()
       SourceLocation Loc;
       if (Err.getLineNo() > 0) {