Add missing Sendable requirements and a conformance

Author: DougGregor

stdlib/public/Concurrency/AsyncStreamBuffer.swift

@@ -543,7 +543,7 @@ extension AsyncThrowingStream {
 }
 
 // this is used to store closures; which are two words
-final class _AsyncStreamCriticalStorage<Contents> {
+final class _AsyncStreamCriticalStorage<Contents>: UnsafeSendable {
   var _value: Contents
   private init(_doNotCallMe: ()) {
     fatalError("_AsyncStreamCriticalStorage must be initialized by create")

stdlib/public/Concurrency/MainActor.swift

@@ -45,7 +45,7 @@ import Swift
 @available(SwiftStdlib 5.5, *)
 extension MainActor {
   /// Execute the given body closure on the main actor.
-  public static func run<T>(
+  public static func run<T: Sendable>(
     resultType: T.Type = T.self,
     body: @MainActor @Sendable () throws -> T
   ) async rethrows -> T {

stdlib/public/Concurrency/SourceCompatibilityShims.swift

@@ -155,11 +155,11 @@ public func async<T>(
 @available(SwiftStdlib 5.5, *)
 extension Task where Success == Never, Failure == Never {
   @available(*, deprecated, message: "`Task.Group` was replaced by `ThrowingTaskGroup` and `TaskGroup` and will be removed shortly.")
-  public typealias Group<TaskResult> = ThrowingTaskGroup<TaskResult, Error>
+  public typealias Group<TaskResult: Sendable> = ThrowingTaskGroup<TaskResult, Error>
 
   @available(*, deprecated, message: "`Task.withGroup` was replaced by `withThrowingTaskGroup` and `withTaskGroup` and will be removed shortly.")
   @_alwaysEmitIntoClient
-  public static func withGroup<TaskResult, BodyResult>(
+  public static func withGroup<TaskResult: Sendable, BodyResult>(
       resultType: TaskResult.Type,
       returning returnType: BodyResult.Type = BodyResult.self,
       body: (inout Task.Group<TaskResult>) async throws -> BodyResult

stdlib/public/Concurrency/Task.swift

@@ -756,7 +756,7 @@ func _enqueueJobGlobalWithDelay(_ delay: UInt64, _ task: Builtin.Job)
 public func _asyncMainDrainQueue() -> Never
 
 @available(SwiftStdlib 5.5, *)
-public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
+public func _runAsyncMain(@_unsafeSendable _ asyncFun: @escaping () async throws -> ()) {
   Task.detached {
     do {
 #if !os(Windows)