UnsafePointer Sendable should be revoked

[Douglas_Gregor]

@xwu

I had thought there was discussion about marking an explicit Sendable conformance as unavailable?

I brought it up in the thread on staging Sendable checking, with this suggestion:

public struct MyType : @available(unavailable, *) Sendable {
  var x, y: Double
  // some day we may add some shared state here
}

Doug