MSVC2012 does not choose to use uint64_t as underlying type without this.

MSVC2012 seems to choose int as the underlying type for an enum even if one of
its member is unsigned long long.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200519 91177308-0d34-0410-b5e6-96231b3b80d8