Factor the option and checking of compiler version better. Put the
option with the others in the top level CMakeLists, and put the check in
HandleLLVMOptions. This will also let it be used from the standalone
Clang builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199149 91177308-0d34-0410-b5e6-96231b3b80d8