Explicit Sendable conformances for @frozen types

Task groups and `UnsafeCurrentTask` are non-`Sendable`.
`UnownedSerialExecutor` is always `Sendable`.

Fixes rdar://86496341.

Author: DougGregor

stdlib/public/Concurrency/Executor.swift

@@ -46,7 +46,7 @@ public protocol SerialExecutor: Executor {
 /// actor.
 @available(SwiftStdlib 5.1, *)
 @frozen
-public struct UnownedSerialExecutor {
+public struct UnownedSerialExecutor: Sendable {
   #if compiler(>=5.5) && $BuiltinExecutor
   @usableFromInline
   internal var executor: Builtin.Executor

stdlib/public/Concurrency/Task.swift

@@ -717,6 +717,10 @@ public struct UnsafeCurrentTask {
   }
 }
 
+@available(SwiftStdlib 5.1, *)
+@available(*, unavailable)
+extension UnsafeCurrentTask: Sendable { }
+
 @available(SwiftStdlib 5.1, *)
 extension UnsafeCurrentTask: Hashable {
   public func hash(into hasher: inout Hasher) {

stdlib/public/Concurrency/TaskGroup.swift

@@ -383,6 +383,10 @@ public struct TaskGroup<ChildTaskResult: Sendable> {
   }
 }
 
+@available(SwiftStdlib 5.1, *)
+@available(*, unavailable)
+extension TaskGroup: Sendable { }
+
 // Implementation note:
 // We are unable to just™ abstract over Failure == Error / Never because of the
 // complicated relationship between `group.spawn` which dictates if `group.next`
@@ -639,6 +643,10 @@ public struct ThrowingTaskGroup<ChildTaskResult: Sendable, Failure: Error> {
   }
 }
 
+@available(SwiftStdlib 5.1, *)
+@available(*, unavailable)
+extension ThrowingTaskGroup: Sendable { }
+
 /// ==== TaskGroup: AsyncSequence ----------------------------------------------
 
 @available(SwiftStdlib 5.1, *)