swift-package-manager

[Build] Fix regression in Xcc handling.

 - Noticed by Chris Bailey (see #507).

 - This also adds at least some test coverage of -Xcc and -Xswiftc.

 - This also fixes it so we don't quite the extra C flags when passing them to
   Clang (which would still work, because Clang ignores -Xcc, but was incorrect
   and produced a warning).