[Concurrency] fix a few missing @available annotations

Author: ktoso

test/Concurrency/Runtime/async_taskgroup_asynciterator_semantics.swift

@@ -10,6 +10,7 @@ func boom() async throws -> Int {
   throw Boom()
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 func test_taskGroup_next() async {
   let sum = await withThrowingTaskGroup(of: Int.self, returning: Int.self) { group in
     for n in 1...10 {
@@ -40,6 +41,7 @@ func test_taskGroup_next() async {
   print("result with group.next(): \(sum)")
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 func test_taskGroup_for_in() async {
   let sum = await withThrowingTaskGroup(of: Int.self, returning: Int.self) { group in
     for n in 1...10 {
@@ -70,6 +72,7 @@ func test_taskGroup_for_in() async {
   print("result with for-in: \(sum)")
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 func test_taskGroup_asyncIterator() async {
   let sum = await withThrowingTaskGroup(of: Int.self, returning: Int.self) { group in
     for n in 1...10 {
@@ -107,6 +110,7 @@ func test_taskGroup_asyncIterator() async {
   print("result with async iterator: \(sum)")
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @main struct Main {
   static func main() async {
     await test_taskGroup_next()

test/Concurrency/Runtime/async_taskgroup_is_asyncsequence.swift

@@ -33,6 +33,7 @@ func test_taskGroup_is_asyncSequence() async {
   print("result: \(sum)")
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 func test_throwingTaskGroup_is_asyncSequence() async throws {
   print(#function)
 

test/Concurrency/async_task_groups.swift

@@ -167,6 +167,7 @@ struct SendableTuple2<A: Sendable, B: Sendable>: Sendable {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension Collection where Self: Sendable, Element: Sendable, Self.Index: Sendable {
 
   /// Just another example of how one might use task groups.