SE-0366: Move Function + "Use After Move" Diagnostic

[ksluder (Kyle Sluder)]

@beccadax

But then I realized, do we actually want other functions using __moved? Or do we want functions to use __owned parameters and leave it to their callers to write move(_:), so they can choose whether they want moving or copying semantics on a per-call basis? On reflection, I think we want the latter—it’s more flexible and more explicit.

One benefit to a move/__moved parameter modifier is that it provides an obvious spelling for initializers for move-only wrapper types:

/// A stdlib-provided replacement for all the adhoc @unchecked Sendable wrappers people are writing
struct Envelope<T> : MoveOnly, Sendable {
  var wrappedValue: T
  init(wrapping value: move T) {
    wrappedValue = move(value)
  }
}

(certain other details of this implementation assumed/omitted)