swift234

[ClangImporter] Allow importing modules even if there's a fatal Clang error.

This was supposed to prevent crashes, but it ended up /causing/ crashes
because we may hit a fatal Clang error /while/ importing Clang modules
and bridging headers.

I left the test on the logic that imports bridging headers because that
happens under more controlled circumstances. (Clang modules that have
Swift counterparts should never have bridging headers.)

rdar://problem/21689014

Swift SVN r30512