[modules] Change the way we deal with .d output for explicitly-specified module

files: include the .pcm file itself in the .d output, rather than including its
own input files. Other forms of module file continue to be transparent for .d
output.

Arguably, the input files for the .pcm file are still inputs to the
compilation, but that's unnecessary for make-like build systems (where the
mtime of the .pcm file is sufficient) and harmful for smarter build systems
that know about module files and want to track only the local dependencies.

llvm-svn: 244923