[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

llvm-svn: 335570

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4836,11 +4836,14 @@ void ModuleSummaryIndexBitcodeReader::setValueGUID(
   if (PrintSummaryGUIDs)
     dbgs() << "GUID " << ValueGUID << "(" << OriginalNameID << ") is "
            << ValueName << "\n";
-  
+
   // UseStrtab is false for legacy summary formats and value names are
-  // created on stack. We can't use them outside of parseValueSymbolTable.
+  // created on stack. In that case we save the name in a string saver in
+  // the index so that the value name can be recorded.
   ValueIdToValueInfoMap[ValueID] = std::make_pair(
-      TheIndex.getOrInsertValueInfo(ValueGUID, UseStrtab ? ValueName : ""),
+      TheIndex.getOrInsertValueInfo(
+          ValueGUID,
+          UseStrtab ? ValueName : TheIndex.saveString(ValueName.str())),
       OriginalNameID);
 }