[Codegen] If reasonable, materialize clang's `AllocAlignAttr` as llvm's...

[Codegen] If reasonable, materialize clang's `AllocAlignAttr` as llvm's Alignment Attribute on call-site function return value

Summary:
Much like with the previous patch (D73005) with `AssumeAlignedAttr`
handling, results in mildly more readable IR,
and will improve test coverage in upcoming patch.

Note that in `AllocAlignAttr`'s case, there is no requirement
for that alignment parameter to end up being an I-C-E.

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

Reviewed By: erichkeane

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73006