clang/Driver: [PR12920] Don't forward any -W arguments to gcc-as and gcc-ld.

AFAIK, there are no -W options for gcc-as and gcc-ld.
It caused failure to build clang with gcc-4.7 on cygwin.

FIXME: Could we recategorize Options for gcc-as and gcc-ld?
llvm-svn: 188668