[Sema] Don't disallow placing `__attribute__((alloc_align(param_idx)))` on...

[Sema] Don't disallow placing `__attribute__((alloc_align(param_idx)))` on `std::align_val_t`-typed parameters

Summary:
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

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73019