[ThinLTO/lld] Document constant bool ModuleSummaryIndex parameter (NFC)

Makes this consistent with other ModuleSummaryIndex constructor calls.

llvm-svn: 334141
diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
index ee55e7c..168bfc9 100644
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -208,7 +208,7 @@
   if (!OS)
     return;
 
-  ModuleSummaryIndex M(false);
+  ModuleSummaryIndex M(/*HaveGVs*/ false);
   M.setSkipModuleByDistributedBackend();
   WriteIndexToFile(M, *OS);