[ThinLTO] Double bits of module hash used for renaming

Summary:
Use 64 instead of 32 bits of the module hash as the suffix when renaming
after promotion to reduce the likelihood of a collision (which we
observed in a binary when using 32 bits).

Reviewers: pcc

Subscribers: llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D39443

llvm-svn: 316996

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -743,7 +743,8 @@ class ModuleSummaryIndex {
   static std::string getGlobalNameForLocal(StringRef Name, ModuleHash ModHash) {
     SmallString<256> NewName(Name);
     NewName += ".llvm.";
-    NewName += utostr(ModHash[0]); // Take the first 32 bits
+    NewName += utostr((uint64_t(ModHash[0]) << 32) |
+                      ModHash[1]); // Take the first 64 bits
     return NewName.str();
   }