[ThinLTO] Add string saver onto index for value names

Summary:
Adds a string saver to the ModuleSummaryIndex so it can store value
names in the case of adding a ValueInfo for a GUID when we don't
have the name stored in a Module string table. This is motivated
by the upcoming summary parser patch, where we will read value names
from the summary entry and want to store them, even when a Module
is not available.

Currently this allows us to store the name in the legacy bitcode case,
and I have added a test to show that.

Reviewers: pcc, dexonsmith

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D47842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335570 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ModuleSummaryAnalysis.cpp

@@ -607,14 +607,14 @@ ModuleSummaryIndexWrapperPass::ModuleSummaryIndexWrapperPass()
 
 bool ModuleSummaryIndexWrapperPass::runOnModule(Module &M) {
   auto &PSI = *getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
-  Index = buildModuleSummaryIndex(
+  Index.emplace(buildModuleSummaryIndex(
       M,
       [this](const Function &F) {
         return &(this->getAnalysis<BlockFrequencyInfoWrapperPass>(
                          *const_cast<Function *>(&F))
                      .getBFI());
       },
-      &PSI);
+      &PSI));
   return false;
 }