Remove __VERSION__

Summary:
It has been introduced in 2011 for gcc compat:
https://github.com/llvm-mirror/clang/commit/ad1a4c6e89594e704775ddb6b036ac982fd68cad
it is probably time to remove it


Reviewers: rnk, dexonsmith

Reviewed By: rnk

Subscribers: dschuff, aheejin, fedor.sergeev, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64062

llvm-svn: 365962