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

This reverts commit r345803 and r345915 (a follow-up fix to r345803).

Reason: r345803 blocks our internal integrate because of the new
warnings showing up in too many places. The fix is actually correct,
we will reland it after figuring out how to integrate properly.

llvm-svn: 345963