Bring back TaskGroup.next() and ThrowingTaskGroup.next() as public API

With the introduction of `next(isolation:)` into the task group types,
we removed the public APIs for the no-argument `next()` versions,
leaving them only as `@usableFromInline internal` entrypoints for the
ABI. Because the `next(isolation:)` versions have a default argument,
this was sufficient for providing source compatibility for calls, but
not for protocol conformances.

Bring these APIs back publicly. Fixes rdar://127499568.

Author: DougGregor

stdlib/public/Concurrency/TaskGroup.swift

@@ -574,10 +574,8 @@ public struct TaskGroup<ChildTaskResult: Sendable> {
     return try! await _taskGroupWaitNext(group: _group) // !-safe cannot throw, we're a non-throwing TaskGroup
   }
 
-  @usableFromInline
   @available(SwiftStdlib 5.1, *)
-  @_silgen_name("$sScG4nextxSgyYaF")
-  internal mutating func __abi_next() async -> ChildTaskResult? {
+  public mutating func next() async -> ChildTaskResult? {
     // try!-safe because this function only exists for Failure == Never,
     // and as such, it is impossible to spawn a throwing child task.
     return try! await _taskGroupWaitNext(group: _group) // !-safe cannot throw, we're a non-throwing TaskGroup
@@ -1035,10 +1033,8 @@ public struct ThrowingTaskGroup<ChildTaskResult: Sendable, Failure: Error> {
     return try await _taskGroupWaitNext(group: _group)
   }
 
-  @usableFromInline
   @available(SwiftStdlib 5.1, *)
-  @_silgen_name("$sScg4nextxSgyYaKF")
-  internal mutating func __abi_next() async throws -> ChildTaskResult? {
+  public mutating func next() async throws -> ChildTaskResult? {
     return try await _taskGroupWaitNext(group: _group)
   }
 

test/Concurrency/async_task_groups_as_sequence.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify -strict-concurrency=targeted
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify -strict-concurrency=complete
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify -strict-concurrency=complete -enable-upcoming-feature RegionBasedIsolation
+
+// REQUIRES: concurrency
+// REQUIRES: asserts
+// REQUIRES: libdispatch
+
+@available(SwiftStdlib 5.1, *)
+@rethrows
+protocol TGP: AsyncSequence, AsyncIteratorProtocol { }
+
+@available(SwiftStdlib 5.1, *)
+extension TaskGroup: TGP { }
+// expected-warning@-1{{extension declares a conformance of imported type 'TaskGroup' to imported protocol 'AsyncIteratorProtocol'}}
+// expected-note@-2{{add '@retroactive' to silence this warning}}
+
+@available(SwiftStdlib 5.1, *)
+extension ThrowingTaskGroup: TGP { }
+// expected-warning@-1{{extension declares a conformance of imported type 'ThrowingTaskGroup' to imported protocol 'AsyncIteratorProtocol'}}
+// expected-note@-2{{add '@retroactive' to silence this warning}}