swift234

[ClangImporter] Don't try to recover if an Objective-C module can't be loaded.

This is already a fatal error, so it doesn't matter how many diagnostics we
dump after that. It's better not to lie to the rest of the compiler about
whether the load succeeds.

No immediate Radar, but this fixes a crash in the test case I'm about to
commit, and possibly some other mystery "dependency missing" crashes in
XREF resolution.

Swift SVN r20919