GCC doesn't accept (void) to silence warn_unused_result.

This is stupid, but the GCC maintainers refused to change it:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

For some reason, KJ's uses of (void) did not warn with GCC 5 but do warn with GCC 7. Supposedly, GCC *never* supported silencing with (void), so there must have been some other bug that caused GCC to fail to trigger the warning previously -- maybe related to the fact that the values being returned are non-trivial types?

C++17 introduces `[[nodiscard]]` which is defined as being squelchable using `(void)`, but we're still on C++14, and KJ_UNUSED_RESULT is a post-declaration attribute so can't be defined in terms of the new C++17 attribute even if the compiler supports it. Sigh.

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

@@ -631,7 +631,7 @@ TEST(Async, TaskSet) {
     EXPECT_EQ(2, counter++);
   }));
 
-  (void)evalLater([&]() {
+  auto ignore KJ_UNUSED = evalLater([&]() {
     KJ_FAIL_EXPECT("Promise without waiter shouldn't execute.");
   });
 
@@ -725,7 +725,10 @@ TEST(Async, Detach) {
   bool ran2 = false;
   bool ran3 = false;
 
-  (void)evalLater([&]() { ran1 = true; });  // let returned promise be destroyed (canceled)
+  {
+    // let returned promise be destroyed (canceled)
+    auto ignore KJ_UNUSED = evalLater([&]() { ran1 = true; });
+  }
   evalLater([&]() { ran2 = true; }).detach([](kj::Exception&&) { ADD_FAILURE(); });
   evalLater([]() { KJ_FAIL_ASSERT("foo"){break;} }).detach([&](kj::Exception&& e) { ran3 = true; });
 

c++/src/kj/compat/http-test.c++

@@ -887,7 +887,9 @@ KJ_TEST("HttpClient canceled write") {
     auto req = client->request(HttpMethod::POST, "/", HttpHeaders(table), uint64_t(4096));
 
     // Start a write and immediately cancel it.
-    (void)req.body->write(body.begin(), body.size());
+    {
+      auto ignore KJ_UNUSED = req.body->write(body.begin(), body.size());
+    }
 
     KJ_EXPECT_THROW_MESSAGE("overwrote", req.body->write("foo", 3).wait(waitScope));
     req.body = nullptr;