Update __VERSION__ to remove the hardcoded 4.2.1 version

Summary:
Just like in https://reviews.llvm.org/D56803
for -dumpversion

Reviewers: rnk

Reviewed By: rnk

Subscribers: dexonsmith, lebedev.ri, hubert.reinterpretcast, xbolva00, fedor.sergeev, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63048

llvm-svn: 366091