[OpenCL] Warn about side effects for unevaluated vec_step arg

The argument to the `vec_step` builtin is not evaluated.  Hoist the
diagnostic for this in `Sema::CheckUnaryExprOrTypeTraitOperand` such
that it comes before `Sema::CheckVecStepTraitOperandType`.

A minor side-effect of this change is that it also produces the
warning for `co_await` and `co_yield` as `sizeof` arguments now, which
seems to be reasonable given that the warning is emitted for `typeid`
already.

Differential Revision: https://reviews.llvm.org/D91348