We use the just-built clang without any -target or -sdk flags. Keep things
simple and limit the test to macOS for now.
Fixes <rdar://problem/50586614>.
Specifically, when the class is referenced from Objective-C first,
before being referenced from Swift in any way. This is important for
resilience, since up until now Objective-C isn't used to the size of a
class changing based on anything other than a superclass's size
changing.
This is basically the test that would have gone with 9024768b0 had
there been a build of libobjc to test it with.
rdar://problem/45718008
