swift234

[PrintAsObjC] Forward-declare things for the generated header when possible...

...and just outright import the bridging header if that's what's needed.

This means we'll use @class and @protocol whenever we're just using a class
or protocol in a type, but still import the enclosing module when we need
the definition. We'll also fall back to the module (or bridging header) if
we need something /else/ from C: a struct, a typedef, whatever.

<rdar://problem/17183425>

Swift SVN r18795