Move Capability::Client::whenResolved() out-of-line to make MSVC linker happy.

It seems like MSVC is generating this function in translation units where it isn't acutally called, which inadvertently causes non-RPC Cap'n Proto code to depend on kj-async.

c++/src/capnp/capability.c++

@@ -121,6 +121,10 @@ kj::Promise<void> ClientHook::whenResolved() {
   }
 }
 
+kj::Promise<void> Capability::Client::whenResolved() {
+  return hook->whenResolved().attach(hook->addRef());
+}
+
 // =======================================================================================
 
 static inline uint firstSegmentSize(kj::Maybe<MessageSize> sizeHint) {

c++/src/capnp/capability.h

@@ -898,9 +898,6 @@ template <typename T>
 inline typename T::Client Capability::Client::castAs() {
   return typename T::Client(hook->addRef());
 }
-inline kj::Promise<void> Capability::Client::whenResolved() {
-  return hook->whenResolved().attach(hook->addRef());
-}
 inline Request<AnyPointer, AnyPointer> Capability::Client::typelessRequest(
     uint64_t interfaceId, uint16_t methodId,
     kj::Maybe<MessageSize> sizeHint) {