[concurrency] add resume<E: Error>(with: Result<T, E>) to UnsafeThrowingContinuation

A handy overload that allows one to return a Result<> to
a continuation without needing to manually unwrap it in client code.

Resolves rdar://71870249

Author: kavon

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -47,6 +47,15 @@ public struct UnsafeThrowingContinuation<T> {
   public func resume(returning: __owned T)
   @_silgen_name("swift_continuation_throwingResumeWithError")
   public func resume(throwing: __owned Error)
+
+  public func resume<E: Error>(with result: Result<T, E>) {
+    switch result {
+      case .success(let val):
+        self.resume(returning: val)
+      case .failure(let err):
+        self.resume(throwing: err)
+    }
+  }
 }
 
 #if _runtime(_ObjC)

test/Concurrency/async_tasks.swift

@@ -62,6 +62,16 @@ func test_unsafeThrowingContinuations() async {
     continuation.resume(throwing: MyError())
   }
 
+  // using resume(with:)
+  let _: String = try await withUnsafeThrowingContinuation { continuation in
+    let result : Result<String, MyError> = .success("")
+    continuation.resume(with: result)
+  }
+
+  let _: String = try await withUnsafeThrowingContinuation { continuation in
+    continuation.resume(with: .failure(MyError()))
+  }
+
   // TODO: Potentially could offer some warnings if we know that a continuation was resumed or escaped at all in a closure?
 }