[LLVM][NFC] Adding an Alignment type to LLVM

Summary:
This patch introduces a type to straighten LLVM's alignment management.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html

The next step is to use this type throughout LLVM

Reviewers: jfb, jakehehrlich

Subscribers: mgorny, mgrang, dexonsmith, llvm-commits, courbet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64790

llvm-svn: 367393