swift234

Don't try to emit SIL for external definitions that
haven't been type-checked.

Deserialization and so on can cause the creation of
synthetic external definitions after the type-checker
has finished processing.  I hope we don't need them?
This really doesn't seem like the right fix, but the
alternative is a non-deterministic crash when emitting
a function body with no types.  I haven't figured out
why it's non-deterministic.

Speculatively committing this while awaiting local
test suite verification.

Swift SVN r23734