Add @_implicitSelfCapture to asyncDetached.

Like `async`, `asyncDetached` should not require `self.` because it is
unlikely to introduce cycles.

(cherry picked from commit 2000bf9)

Author: DougGregor

stdlib/public/Concurrency/Task.swift

@@ -489,7 +489,7 @@ public func detach<T>(
 @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public func asyncDetached<T>(
   priority: Task.Priority = .unspecified,
-  operation: __owned @Sendable @escaping () async -> T
+  @_implicitSelfCapture operation: __owned @Sendable @escaping () async -> T
 ) -> Task.Handle<T, Never> {
   return detach(priority: priority, operation: operation)
 }
@@ -499,7 +499,7 @@ public func asyncDetached<T>(
 @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public func asyncDetached<T>(
   priority: Task.Priority = .unspecified,
-  operation: __owned @Sendable @escaping () async throws -> T
+  @_implicitSelfCapture operation: __owned @Sendable @escaping () async throws -> T
 ) -> Task.Handle<T, Error> {
   return detach(priority: priority, operation: operation)
 }