swift234

Infer "dynamic" for Objective-C methods introduced in class extensions.

When we introduce a method into a class extension (such that it does
not override any other method), we cannot add a vtable slot for that
method and therefore cannot perform dynamic dispatch on it without
going through objc_msgSend. To work around this bug
(<rdar://problem/17950064>), infer 'dynamic' for these cases, which
pushes us through objc_msgSend to get the appropriate semantics.

Note that we do *not* want this to interfere with inference of
'final', which means allowing 'final' inference to delete the
'dynamic' we introduce with the above.

Swift SVN r21135