[Sema] CheckTautologicalComparisonWithZero(): always complain about enums

Hopefully fixes test-clang-msc-x64-on-i686-linux-RA build.

The underlying problem is that the enum is signed there.
Yet still, it is invalid for it to contain negative values,
so the comparison is always tautological in this case.

No differential, but related to https://reviews.llvm.org/D37629

llvm-svn: 313747