[5.7][stdlib] Fix warning on Clock.Duration

[lorentey]

(Cherry picked from #58411, reviewed by @phausler, @slavapestov & @stephentyrone)

#42314 introduced Clock.Duration as an associated type:

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

Unfortunately this produces a warning:

…/swift/stdlib/public/Concurrency/Clock.swift:35:28: warning: redundant conformance constraint 'Self.Duration' : 'DurationProtocol'
  associatedtype Duration: DurationProtocol
                           ^

To keep the stdlib build log free of noise, it seems we’ll need to update this declaration to remove the direct requirement.

rdar://92306564

[lorentey]

@swift-ci test