[CSApply] Fix a KeyPath crash in SIL when base type is AnyObject

[theblixguy]

This PR fixes a SIL crash when the base type of a Swift KeyPath is AnyObject. For example:

import UIKit

class Foo: NSObject {
  func crash() {
    let kp = \AnyObject.accessibilityFrame
  }
}

Resolves SR-10146.

[theblixguy]

cc @jckarter @slavapestov

[jckarter]

Thanks!

[jckarter]

@swift-ci Please test

[theblixguy]

Seems like the tests never ran

[jckarter]

@swift-ci Please test

[jckarter]

@slavapestov raised the point that it's not a good idea to diagnose in CSApply, because we may apply multiple solutions in the face of overloading or diagnostic recovery. If we could either reject this solution in the constraint solver or in CSDiag that would be more robust.

[theblixguy]

@jckarter Hmm we could also probably reject it in CSGen in visitKeyPathExpr as well.

I’ll create a new PR shortly to move this to the solver.

[slavapestov]

@theblixguy I think your best bet is to reject it in the solver because you won't necessarily know the base type of the key path in CSGen. For example if I use \.dynamicMember in a context that's expecting a KeyPath<T> and T is later inferred as AnyObject. Maybe I'm missing something though.