[SR-15694] make isolation inference + override more consistent

[kavon]

During actor isolation inference, we would unconditionally choose the
isolation of the overridden decl (when, say, there is no attribute on the decl).
The overridden decl is identified with getOverriddenDecl.

This works mostly fine, but initializers have some unusual overridden decls
returned by that method. For example, in the following code:

@objc class PictureFrame: NSObject {
  init(size: Int) { }
}

@MainActor
class FooFrame: PictureFrame {
  init() {
    super.init(size: 0)
  }
}

that method claims that FooFrame.init() overrides PictureFrame.init(), when
it really does not! So, if we were to unconditionally take the isolation from
PictureFrame.init() (and thus NSObject.init()), then we'd infer that
FooFrame.init() has unspecified isolation, despite the nominal it resides in
being marked as MainActor. This is in essence the problem in SR-15694, where
placing the isolation directly on the initializer fixes this issue.

If FooFrame.init() really does override, then why can it be MainActor? Well,
we have a rule in one part of the type-checker saying that if an ObjC-imported
decl has unspecified isolation, then overriding it with isolation is permitted.
But the other part of the type-checker dealing with the isolation inference was
not permitting that.

So, this patch unifies how actor-isolation inference is conducted to reuse that
same logic. In addition, the inference now effectively says that, if the decl
has no inferred isolation, or the inferred isolation is invalid according to the
overriding rules, then we use the isolation of the overridden decl. This
preserves the old behavior of the inference, while also fixing this issue with
ObjC declarations by expanding where isolation is allowed.

For example, as a consequence of this change, in the following code:

@MainActor
class FooFrame: NotIsolatedPictureFrame {
  override func rotate() {
    mainActorFn()
  }
}

if NotIsolatedPictureFrame is a plain-old not-isolated class imported from
Objective-C, then rotate will now have MainActor isolation, instead of
having unspecified isolation like it was inferred to have previously. This
helps make things consistent, because rotate is allowed to be @MainActor if
it were explicitly marked as such.

resolves rdar://87217618 / SR-15694

[kavon]

@swift-ci please test

[DougGregor]

This seems like the kind of thing we should diagnose with -warn-concurrency and in Swift 6, because it is a hole in the safety model. That can be a follow-on PR, of course.

[kavon]

@swift-ci please test and merge

[kavon]

@swift-ci please test and merge