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

This broke one of the builds, presumably side-by-side modular CMake.
Investigating.

This reverts commit r212998.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213005 91177308-0d34-0410-b5e6-96231b3b80d8