Remove CMake-Xcode hack that symlinked llvm-config into a common place.

When building Clang separately from LLVM with CMake, one should set
the path of llvm-config via the cache variable LLVM_CONFIG.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198316 91177308-0d34-0410-b5e6-96231b3b80d8