Add call to find_package to load LLVM dependencies

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

llvm-svn: 261290