Reapply "Fix regression in behavior of clang -x c++-header -fmodule-name=XXX"

This reverts commit r345963. We have a path forward now.

Original commit message:
The driver accidentally stopped passing the input filenames on to -cc1
in this mode due to confusion over what action was being requested.

This change also fixes a couple of crashes I encountered when passing
multiple files to such a -cc1 invocation.

llvm-svn: 346130