Write isUInt using template specializations to work around an incorrect MSVC warning.

Summary:
Per D22441, MSVC warns on our old implementation of isUInt<64>.  It sees
uint64_t(1) << 64 and doesn't realize that it's not going to be
executed.  Writing as a template specialization is ugly, but prevents
the warning.

Reviewers: RKSimon

Subscribers: majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D22472

llvm-svn: 275909