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

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

[forum]: https://forums.swift.org/t/pitch-primary-associated-types-in-the-standard-library/56426/39

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

Author: lorentey

stdlib/public/Concurrency/Clock.swift

@@ -32,7 +32,8 @@ import Swift
 /// `SuspendingClock`.
 @available(SwiftStdlib 5.7, *)
 public protocol Clock: Sendable {
-  associatedtype Instant: InstantProtocol
+  associatedtype Duration: DurationProtocol
+  associatedtype Instant: InstantProtocol where Instant.Duration == Duration
 
   var now: Instant { get }
   var minimumResolution: Instant.Duration { get }