[ThinLTO] Rename index IsAnalysis flag to HaveGVs (NFC)

With the upcoming patch to add summary parsing support, IsAnalysis would
be true in contexts where we are not performing module summary analysis.
Rename to the more specific and approprate HaveGVs, which is essentially
what this flag is indicating.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334140 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ModuleSummaryAnalysis.cpp

@@ -408,7 +408,7 @@ ModuleSummaryIndex llvm::buildModuleSummaryIndex(
     std::function<BlockFrequencyInfo *(const Function &F)> GetBFICallback,
     ProfileSummaryInfo *PSI) {
   assert(PSI);
-  ModuleSummaryIndex Index(/*IsPerformingAnalysis=*/true);
+  ModuleSummaryIndex Index(/*HaveGVs=*/true);
 
   // Identify the local values in the llvm.used and llvm.compiler.used sets,
   // which should not be exported as they would then require renaming and