[Alignment] Migrate Attribute::getWith(Stack)Alignment

[Alignment] Migrate Attribute::getWith(Stack)Alignment

Summary:
This is patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790

Reviewers: courbet, jdoerfert

Reviewed By: courbet

Subscribers: arsenm, jvesely, nhaehnle, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D68792

llvm-svn: 374884