It's likely that these were listed as "REQUIRES: OS=macosx" to not
redundantly run them for iOS et al, but they don't take that long and
so it's more useful for Linux devs to be able to run them locally if
need be. Or to catch something that really is different on non-macOS.
Instead of visiting all members of all types and extensions, bail out
early if the type is not a class or protocol, or the extension is not
extending a class. This means we don't visit structs, enums or
protocol extensions at all, which will avoid delayed parsing.
Also, we were evaluating isObjC() on each member, which is an expensive
operation; if the member does not have an explicit @objc we would still
have to check if it overrides an @objc method or witnesses an @objc
protocol requirement.
Since most members are not ever found by dynamic lookup, this is wasted
work. Instead, let's rely on AnyObject lookup filtering non-@objc
members at the call site, which it was already doing anyway.
