Isolated conformance can be escaped by coercing metatype to protocol

[hamishknight]

The following "escapes" the MainActor isolated conformance of K to P, incorrectly allowing it to be used in a non-isolated context:

protocol P {
  static func foo()
}
struct S<T: P>: P {
  static func foo() { T.foo() }
}
@MainActor struct K: @MainActor P {
  static func foo() {
    MainActor.preconditionIsolated("💥")
  }
}
await Task.detached {
  let x = S<K>.self
  (x as P.Type).foo()
}.value