[stdlib] Add Clock.Duration as an associated type requirement

[lorentey]

As discussed on the forum, we'll likely want to use Duration as the primary associated type on protocol Clock; however, that protocol currently only has Instant.

To support declaring it as the primary associated type, protocol Clock needs to have Duration as an associated type requirement:

@available(SwiftStdlib 5.7, *)
public protocol Clock: Sendable {
  associatedtype Duration: DurationProtocol
  associatedtype Instant: InstantProtocol where Instant.Duration == Duration

  var now: Instant { get }
  var minimumResolution: Instant.Duration { get }

  func sleep(until deadline: Instant, tolerance: Instant.Duration?) async throws
}

This setup is reminiscent of Sequence.Element vs. Sequence.Iterator.Element.

SE-0329 has not yet shipped in an ABI stable release, so we still have the opportunity to address this.

rdar://91591545

[lorentey]

@swift-ci test

[lorentey]

We could also update references to Instant.Duration elsewhere in the protocol & its extensions. Do we want to do that?

var minimumResolution: Instant.Duration { get }

  @available(SwiftStdlib 5.7, *)
  public func measure(_ work: () throws -> Void) rethrows -> Instant.Duration {

[phausler]

Since the code-hints will complete to a potentially ambiguous type it likely is good to just leave them as Instant.Duration (plus it is less work to do).

[phausler]

Looks good to me!

[lorentey]

swiftlang/swift-evolution#1618 is merged now; this looks OK to land.