swift234

[Driver] Be more explicit about the inputs to the merge-module action.

Previously we treated this the same as the inputs to the linker, but this
was problematic for a few reasons:
- Backend jobs don't produce
- .o inputs never produce .swiftmodule files (obviously).

So now we:
- explicitly track what can produce a swiftmodule
- don't run module merging if there's nothing to merge
- allow linking without a swiftmodule even under -g

Which gets us closer to being able to run the entire test suite with -g.

rdar://problem/22332569

Swift SVN r31376