[ThinLTO] Fix gcc warnings from commit

Remove an unnecessary const from an Optional return type introduced
in r364960 that gcc 7.4.0 warns about. It is unnecessary and possibly
incorrect.

llvm-svn: 365047

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -1308,7 +1308,7 @@ public:
 
   /// For the given \p TypeId, this returns the TypeIdCompatibleVtableMap
   /// entry if present in the summary map. This may be used when importing.
-  Optional<const TypeIdCompatibleVtableInfo>
+  Optional<TypeIdCompatibleVtableInfo>
   getTypeIdCompatibleVtableSummary(StringRef TypeId) const {
     auto I = TypeIdCompatibleVtableMap.find(TypeId);
     if (I == TypeIdCompatibleVtableMap.end())