Modules: Fix implicit output file for .cppm to .pcm instead of stdout

This code was introduced back in r178148, a change to introduce
-module-file-info - which still exists & seems like it's still tested (&
this change didn't cause any of those tests to fail).

It doesn't look like this change was necessary there - since it's about
pcm output, whereas -module-file-info looks like it's for pcm /input/.
So I'm not really sure what the original motivation was.

I'm open to ideas though, if it turns out the original change was
necessary/useful.

llvm-svn: 334778