Sema: Handle C11 atomics when diagnosing out of range comparisons

This fixes a couple of asserts when analyzing comparisons involving
C11 atomics that were uncovered by r205608 when we extended the
applicability of -Wtautological-constant-out-of-range-compare.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213573 91177308-0d34-0410-b5e6-96231b3b80d8