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

Authored by lebedev.ri on Jan 23 2020, 11:50 AM.


[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

Subscribers: cfe-commits

Tags: #clang

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