Add *Continuation.resume() for continuations returning Void.

This change implements the changes proposed in swift-evolution PR #1264.
Existing test coverage should be sufficient here since the added function
simply calls into the existing `resume(returning:)` function.

This change resolves rdar://74031110.

Author: grynspan

stdlib/public/Concurrency/CheckedContinuation.swift

@@ -151,6 +151,22 @@ public struct CheckedContinuation<T> {
   }
 }
 
+extension CheckedContinuation where T == Void {
+  /// Resume the task awaiting the continuation by having it return normally
+  /// from its suspension point.
+  ///
+  /// A continuation must be resumed exactly once. If the continuation has
+  /// already been resumed through this object, then the attempt to resume
+  /// the continuation again will trap.
+  ///
+  /// After `resume` enqueues the task, control is immediately returned to
+  /// the caller. The task will continue executing when its executor is
+  /// able to reschedule it.
+  public func resume() {
+    self.resume(returning: ())
+  }
+}
+
 public func withCheckedContinuation<T>(
     function: String = #function,
     _ body: (CheckedContinuation<T>) -> Void
@@ -244,6 +260,22 @@ public struct CheckedThrowingContinuation<T> {
   }
 }
 
+extension CheckedThrowingContinuation where T == Void {
+  /// Resume the task awaiting the continuation by having it return normally
+  /// from its suspension point.
+  ///
+  /// A continuation must be resumed exactly once. If the continuation has
+  /// already been resumed through this object, then the attempt to resume
+  /// the continuation again will trap.
+  ///
+  /// After `resume` enqueues the task, control is immediately returned to
+  /// the caller. The task will continue executing when its executor is
+  /// able to reschedule it.
+  public func resume() {
+    self.resume(returning: ())
+  }
+}
+
 public func withCheckedThrowingContinuation<T>(
     function: String = #function,
     _ body: (CheckedThrowingContinuation<T>) -> Void

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -34,6 +34,12 @@ public struct UnsafeContinuation<T> {
   public func resume(returning value: __owned T)
 }
 
+extension UnsafeContinuation where T == Void {
+  public func resume() {
+      self.resume(returning: ())
+  }
+}
+
 @frozen
 public struct UnsafeThrowingContinuation<T> {
   @usableFromInline internal var context: Builtin.RawUnsafeContinuation
@@ -58,6 +64,12 @@ public struct UnsafeThrowingContinuation<T> {
   }
 }
 
+extension UnsafeThrowingContinuation where T == Void {
+  public func resume() {
+      self.resume(returning: ())
+  }
+}
+
 #if _runtime(_ObjC)
 
 // Intrinsics used by SILGen to resume or fail continuations