Because extensions don't have any identity we can check against, we can't
tell when we see an Objective-C category if it came from a Swift extension.
Change PrintAsObjC to mark all such categories with SWIFT_EXTENSION, and
just skip them unilaterally when importing Objective-C code.
Also, actually give Swift extensions a name when writing them as Objective-C
categories. Previously, they were nameless categories ("class extensions"),
but methods in a class extension are supposed to be implemented in the class's
main @implementation, so people were getting unexpected warnings about missing
implementations.
<rdar://problem/17342287>
Swift SVN r19116
