[ADT] Be less clever when using a nonce type for disambiguation.

Old compilers don't like constexpr, but we're only going to use this in one
place anyway: this file. Everyone else should go through PointerLikeTypeTraits.

Update to r261259.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261268 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed