clang -dumpversion returns 4.2.1 for legacy reason, update it

Summary:
It has been introduced in 2011 for gcc compat:
https://github.com/llvm-mirror/clang/commit/95a907fc0f2f717dca38a379c1d2353bfea06d4f
it is probably time to remove it to remove the confusion.

reported: https://bugs.llvm.org/show_bug.cgi?id=38836
https://bugs.launchpad.net/ubuntu/+source/llvm-defaults/+bug/1810860

Reviewers: ddunbar, rnk

Reviewed By: rnk

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D56803

llvm-svn: 356931