SILGen: Handle key paths with @objc optional property and subscript components

[AnthonyLatsis]

Resolves #58337

[AnthonyLatsis]

@swift-ci please smoke test macOS

[AnthonyLatsis]

Note that this will still crash during SIL generation if the optional requirement has a setter. I wonder why optional requirements are even allowed to have a setter if we lack the plumbing for that case.

[jckarter]

Thanks! Yeah, setters on optional properties (and on properties found through dynamic member lookup on AnyObject) were one thing we put off thinking we could come up with a clever way to model, but never got around to. Modeling the key path component as get-only should be sufficient in the meantime.

[jckarter]

@swift-ci Please test

[AnthonyLatsis]

Modeling the key path component as get-only should be sufficient in the meantime.

Will do in a follow-up!

[jckarter]

It looks like the type checker after this change still treats a key path referencing an @objc optional property as a WritableKeyPath, which after this fix, causes us to still crash if people try to do something like foo[keyPath: \.optionalProperty] = bar. Would you be able to fix that aspect as well?

[AnthonyLatsis]

Sorry, got carried away, I'm on it now!