[LLVM][Alignment] Introduce Alignment In Attributes

Summary:
This is patch is part of a serie 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: jfb

Subscribers: hiraditya, dexonsmith, llvm-commits, courbet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65742

llvm-svn: 368002