swift234

[ClangImporter] Drop the "current import" tracking for bridging headers.

Now that bridging headers from ASTs are always present and bridging headers
from the command line are checked in advance, we no longer need to worry
about the Clang instance failing to find the bridging header. That's good,
because this wasn't implemented correctly anyway: if a bridging header
imported a module, we could get an error.

Swift SVN r20129