[5.1] [Typechecker] Allow bridging of Unmanaged to Objective-C for throwing functions

[theblixguy]

Cherry-pick of #24135

---

Previously, we would emit an error diagnostic if a throwing @objc function returned Unmanaged<T>. This is now allowed when T is representable in Objective-C.

For example:

class Foo {}

@objc protocol Bar {
  func throwingMethod1() throws -> Unmanaged<CFArray> // ok
  func throwingMethod2() throws -> Unmanaged<Foo> // error
}

Resolves SR-9035.

[theblixguy]

cc @jrose-apple

[jrose-apple]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 277bb0f

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 277bb0f

[jrose-apple]

:-( Looks like the IR looked different in 5.1. You'll have to update the test.

[theblixguy]

Ah, I didn't expect the IR to be slightly different. No worries, I have updated the test. Could you invoke the CI again? Thanks!

[jrose-apple]

@swift-ci Please test

[jrose-apple]

Thanks, @theblixguy!