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

Prospectively revert to get one of the many build configurations back working
(presumably the side-by-side+modular CMake config).

This reverts commit r212997.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213008 91177308-0d34-0410-b5e6-96231b3b80d8