swift234

Sema: Diagnose overrides of an @objc declaration that cannot be @objc

Now that generic subclasses of @objc classes are supported, dust off
Doug Gregor's fix for <rdar://problem/20385288>. It is now an error
to override an @objc declaration with something that cannot be
represented as @objc.

For example, the override of foo() here will not compile unless
it is explicitly marked @nonobjc:

func foo(i: Int) {}
...
override func foo(i: Int?) {}

Note that I updated IRGen to delete some logic for figuring out when
to emit @objc metadata. We can now rely on Sema to correctly set
isObjC(), instead of checking overrides ourselves. This was wrong
anyway, now that we can have @nonobjc overrides of @objc methods,
and vice versa.

Swift SVN r29263