[Codegen] If reasonable, materialize clang's AllocAlignAttr as llvm's Alignment Attribute on call-site function return value
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
Differential Revision: https://reviews.llvm.org/D73006