Add AIX Version Macros

Summary:
- This patch checks the AIX version and defines the appropriate macros.
- Follow up to a comment on D59048.

Author: andusy

Reviewers: hubert.reinterpretcast, jasonliu, sfertile, xingxue

Reviewed By: sfertile

Subscribers: jsji, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61530

llvm-svn: 360900