Produce errors for unknown options.

This doesn't change a lot since clang still thinks it knows all of the
-f*, -m* and -W* options for example.

Other than the options clang explicitly claims to know, this fixes pr9701.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191249 91177308-0d34-0410-b5e6-96231b3b80d8