This allows a previously-working case of Objective-C forward-declaring
a type in a /different/ Swift module to continue working, as long as
the Swift context being compiled manages to import the other module
properly (including its generated header). This isn't really our
recommended pattern---that would be to @import the module in the
bridging header and forego the forward declaration---but it doesn't
cost much to keep it working. It's also a place where textual and
precompiled bridging headers behaved differently, because precompiled
ones are processed much earlier.
https://bugs.swift.org/browse/SR-3798
