swift234

If imported calls return nil for __nonnull NSString *, pretend it was "".

The string version of r26479. There's a lot of backstory and justification
there, so just read that commit message again. The one addition for String
is that global NSString constants are loaded as String as well, so that
also has to go through the bridging code even though there's no function
call involved.

Finishes rdar://problem/19734621.

Swift SVN r26510