ClangConfig requires LLVMConfig, so add find_package call in
ClangConfig so find_package(clang REQUIRED CONFIG) will just work.  This
makes it easier for cmake based projects to use clang, e.g., tools using
ClangTooling.

Patch by Don Hinton

Differential Revision: http://reviews.llvm.org/D13622

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261290 91177308-0d34-0410-b5e6-96231b3b80d8