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

It turns out this commit was fine. The problem was in the legacy build system (fixed r213010).

This reverts commit r213008.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213014 91177308-0d34-0410-b5e6-96231b3b80d8