swift234

Make -import-underlying-module automatically export that module.

That's how everything behaved anyway. Might as well make it explicit and
stop special-casing it.

I've left in compatibility for modules built with older compilers so that
people using the OS toolchains aren't immediately unable to debug their apps.
As soon as we change the module format in a more significant way, I can take
this out.

Groundwork for rdar://problem/21254367; see next commit.

Swift SVN r29437