[Sema] Try 2: Sanity-check alignment requested via `__attribute__((assume_aligned...

[Sema] Try 2: Sanity-check alignment requested via `__attribute__((assume_aligned(imm)))`

Summary:
For `__builtin_assume_aligned()`, we do validate that the alignment
is not greater than `536870912` (D68824), but we don't do that for
`__attribute__((assume_aligned(N)))` attribute.
I suspect we should.

This was initially committed in a4cfb15d15a8a353fe316f2a9fe1c8c6a6740ef1
but reverted in 210f0882c9e5d6f504b8f29e8a5eae884f812e5c due to
suspicious bot failures.

Reviewers: erichkeane, aaron.ballman, hfinkel, rsmith, jdoerfert

Reviewed By: erichkeane

Subscribers: cfe-commits, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D72994