[clang-tidy] install helper scripts

Scripts are installed in same location as clang-fromat ones, so I think will be good idea to not create dedicated directory.

I checked this patch on my own build on RHEL 6.

Please check it in if it's OK, because I don't have SVN write access.

I think will be good idea to backport this patch to 3.7 release branch.

Probably same should be done for configure build.

Patch by Eugene Zelenko!

Differential revision: http://reviews.llvm.org/D12700

llvm-svn: 247890