Add Promise<T>::ignoreResult() helper for ignoring return value.

This is largely motivated by the fact that Clang currently miscompiles `.then([](auto){})`:

https://llvm.org/bugs/show_bug.cgi?id=24989

c++/src/kj/async-test.c++

@@ -285,6 +285,24 @@ TEST(Async, DeepChain4) {
   promise.wait(waitScope);
 }
 
+TEST(Async, IgnoreResult) {
+  EventLoop loop;
+  WaitScope waitScope(loop);
+
+  bool done = false;
+
+  Promise<void> promise = Promise<int>(123).then([&](int i) {
+    done = true;
+    return i + 321;
+  }).ignoreResult();
+
+  EXPECT_FALSE(done);
+
+  promise.wait(waitScope);
+
+  EXPECT_TRUE(done);
+}
+
 TEST(Async, SeparateFulfiller) {
   EventLoop loop;
   WaitScope waitScope(loop);

c++/src/kj/async.h

@@ -194,6 +194,11 @@ public:
   // actual I/O.  To solve this, use `kj::evalLater()` to yield control; this way, all other events
   // in the queue will get a chance to run before your callback is executed.
 
+  Promise<void> ignoreResult() KJ_WARN_UNUSED_RESULT { return then([](T&&) {}); }
+  // Convenience method to convert the promise to a void promise by ignoring the return value.
+  //
+  // You must still wait on the returned promise if you want the task to execute.
+
   template <typename ErrorFunc>
   Promise<T> catch_(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT;
   // Equivalent to `.then(identityFunc, errorHandler)`, where `identifyFunc` is a function that