handle /clang arguments for clang-cl

[froydnj]

clang 8.0 introduced a better "escape hatch" for clang-cl than -Xclang: /clang:ARG. The former passes the argument directly to cc1; the latter passes the argument to the clang driver itself, essentially allowing anything to be passed through as if you were using clang instead of clang-cl.

I think this is just a matter of adding a SlashClang case to ArgData and adding similar reprocessing of SlashClang arguments as the XClang case already does. But it'd be nice to not completely duplicate the XClang processing code.

[glandium]

There is partial support in #808

[froydnj]

This is done, then.