Don't attempt to run llvm-config in cmake/modules/Makefile when doing
``make clean`` because it won't be available.

This is an attempt to unbreak buildbots broken by r217484.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217490 91177308-0d34-0410-b5e6-96231b3b80d8