swift234

[Driver] Don't crash when an input file has unknown type.

If we can't determine the type of an input file, we assume it's a linker
input of some sort (like Clang does). However, if we're not actually linking,
this resulted in unused inputs, which the driver choked on.

This commit updates the driver to throw away (and diagnose) inputs that
aren't going to be compiled if they aren't going to be used later on.
It also checks that none of our non-object output types are being treated
as linker inputs, since we do have that information around.

<rdar://problem/16019895>

Swift SVN r18667