Reapply r207477 and r207479 without cyclic dependency

Fixed by moving ProcessWarningOptions from Frontend into Basic. All of
the dependencies for ProcessWarningOptions were already in Basic, so
this was a small change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207549 91177308-0d34-0410-b5e6-96231b3b80d8