[modules] Don't pass interesting decls to the consumer for a module file that's

passed on the command line but never actually used. We consider a (top-level)
module to be used if any part of it is imported, either by the current
translation unit, or by any part of a top-level module that is itself used.

(Put another way, a module is used if an implicit modules build would have
loaded its .pcm file.)

llvm-svn: 275481