swift234

[ClangImporter] Avoid doing import work after type checking is done.

I haven't been able to come up with a small reduced test case, and I don't
think I'm likely to in the near future, so I'd like to commit this for now.
This fixes the particular crash reported in rdar://problem/19975256, but
not the general problem. I have a heavier solution for that too, but don't
want to commit it just yet when I don't even have a test for this commit.

rdar://problem/19975256

Swift SVN r27196