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 now.
Associated spec clarification: https://github.com/KhronosGroup/OpenCL-Docs/pull/481