Driver/Frontend: Add support for -mllvm, which forwards options to the LLVM option parser.

 - Note that this is a behavior change, previously -mllvm at the driver level forwarded to clang -cc1. The driver does a little magic to make sure that '-mllvm -disable-llvm-optzns' works correctly, but other users will need to be updated to use -Xclang.

llvm-svn: 101354