[NFC][Codegen] Use MaybeAlign + APInt::getLimitedValue() when creating Alignment attr

Summary: Just an NFC code cleanup i stumbled upon when stumbling through clang alignment attribute handling.

Reviewers: erichkeane, gchatelet, courbet, jdoerfert

Reviewed By: gchatelet

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72993