Move clang feature flags settings out of LLVM core and into cfe

clang r212997 incorporated these settings into its own build system.  They no
longer need to be set from LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212998 91177308-0d34-0410-b5e6-96231b3b80d8