[Sema] Don't disallow placing __attribute__((alloc_align(param_idx))) on std::align_val_t-typed parameters
I kind-of understand why it is restricted to integer-typed arguments,
for general enum's the value passed is not nessesairly the alignment implied,
although one might say that user would know best.
But we clearly should whitelist std::align_val_t,
which is just a thin wrapper over std::size_t,
and is the C++ standard way of specifying alignment.
Reviewers: erichkeane, rsmith, aaron.ballman, jdoerfert
Reviewed By: erichkeane
Differential Revision: https://reviews.llvm.org/D73019