remove redundant LLVM version from version string when setting CLANG_VENDOR

Summary:
When downstream LLVM distributions (like AOSP) set the CLANG_VENDOR
cmake variable, the version string printed by the clang driver looks
like:

$ clang --version
[CLANG_VENDOR] clang version X.X.X ([CLANG_REPOSITORY_STRING] sha) (based on LLVM X.X.X)

Rather than the more standard:
$ clang --version
clang version X.X.X ([CLANG_REPOSITORY_STRING] sha)

Based on feedback the the version string is a little long, the trailing
"(based on LLVM X.X.X)" is redundant and makes less sense after moving
LLVM to the monorepo. And it is only added should vendors set the cmake
variable CLANG_VENDOR. Let's remove it.

Reviewers: jyknight, eli.friedman, rsmith, rjmccall, efriedma

Reviewed By: efriedma

Subscribers: arphaman, efriedma, cfe-commits, srhines

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69925