Fix MSVC -Wmicrosoft-enum-value 'enumerator value is not representable' warning

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288798 91177308-0d34-0410-b5e6-96231b3b80d8

Author: RKSimon

include/llvm/IR/GetElementPtrTypeIterator.h

@@ -34,7 +34,7 @@ namespace llvm {
 
     ItTy OpIt;
     PointerUnion<StructType *, Type *> CurTy;
-    enum { Unbounded = -1ull };
+    enum : uint64_t { Unbounded = -1ull };
     uint64_t NumElements = Unbounded;
     generic_gep_type_iterator() = default;