[sema] Honor explicit global actor isolation for async closures in constraint solver

[gottesmm]

NOTE: This is a draft since I am just testing this out with the source compat suite. I will add more tests/etc if it looks good.

---

Fixes rdar://169803154

Previously, we were emitting an unknown pattern error in code like:

@MainActor
class MainActorIsolatedKlass {
  init() {
    Task.detached { @MainActor in
      _ = self
      for try await x in E.seq {
      }
    }
  }
}

enum E {
  static var seq: some AsyncSequence<Float, Error> {
    AsyncThrowingStream(Float.self) { $0.finish() }
  }
}

The issue was that the closure was considered non-Sendable despite being @mainactor due to the try await in it. Investigation revealed an inconsistency between the constraint solver and getClosureIsolation:

1. In the constraint solver, we were only honoring explicit global actor isolation for non-async functions, which caused us to not infer that the closure should have been Sendable despite the try await.

2. Later getClosureIsolation did the correct thing and we labeled the closure as having global actor isolation, but that did not impact the actual type of the closure being Sendable at the SIL level.

The result was a non-Sendable main actor isolated closure.

This commit removes the restriction that prevented honoring explicit global actor isolation for async closures, making the constraint solver consistent with getClosureIsolation.

[gottesmm]

@swift-ci test source compatibility suite

[gottesmm]

@swift-ci test source compatibility

[xedin]

Release failure is unrelated, it timed out during cleanup.