[clang] [clang][Sema] Handle alloc_align on all HasFunctionProto declarations (PR #210871)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

We should also update `clang/include/clang/Basic/AttrDocs.td` so that the documentation isn't specific to function declarations.

I've added @ahatanak as a reviewer because this is pretty closely related to Objective-C and I want to verify that this attribute is expected to work there (we may have additional work to support the attribute there, or it may be fine).

https://github.com/llvm/llvm-project/pull/210871