[SE-0304] Add (Throwing)?TaskGroup.waitForAll()

Do this as a staged change to the ABI, introducing an underscored
`@usableFromInline` implementation to the ABI that we can rely on
later, and an `@_alwaysEmitIntoClient` version we can inline now.

Author: DougGregor

stdlib/public/Concurrency/TaskGroup.swift

@@ -354,7 +354,14 @@ public struct TaskGroup<ChildTaskResult> {
   internal mutating func awaitAllRemainingTasks() async {
     while let _ = await next() {}
   }
-  
+
+  /// Wait for all remaining tasks in the task group to complete before
+  /// returning.
+  @_alwaysEmitIntoClient
+  public mutating func waitForAll() async {
+    await awaitAllRemainingTasks()
+  }
+
   /// A Boolean value that indicates whether the group has any remaining tasks.
   ///
   /// At the start of the body of a `withTaskGroup(of:returning:body:)` call,
@@ -444,6 +451,17 @@ public struct ThrowingTaskGroup<ChildTaskResult, Failure: Error> {
     }
   }
 
+  public mutating func _waitForAll() async throws {
+    while let _ = try await next() { }
+  }
+
+  /// Wait for all remaining tasks in the task group to complete before
+  /// returning.
+  @_alwaysEmitIntoClient
+  public mutating func waitForAll() async throws {
+    while let _ = try await next() { }
+  }
+
   /// Unconditionally create a child task in the group.
   ///
   /// ### Error handling