Q(Multi)Map: micro-optimize take()

Instead of the RVO-unfriendly¹

    auto r = std::move(it->second);
    erase(it);
    return r;

use map::extract(), if available. The RVO-unfriendly code now only
plagues old-fashioned compilers that, in 2025, didn't, yet, manage to
implement C++17 _cpp_lib_node_extract. The rest enjoys C++17
guaranteed RVO.

Amends 14090760a87f23509b7bb5ad846537c766cb44a5.

¹ theoretically, this code is eligible for NRVO, but embedded in a
  larger function like this, it won't kick in on most compilers.

Reported-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Pick-to: 6.9 6.8
Change-Id: I6654fa7f5ed80d07369eb6a2b37f1e82cd3aa7f0
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qmap.h

@@ -332,12 +332,18 @@ public:
         // elements (the one to be removed can be skipped).
         detach();
 
+#ifdef __cpp_lib_node_extract
+        if (const auto node = d->m.extract(key))
+            return std::move(node.mapped());
+#else
         auto i = d->m.find(key);
         if (i != d->m.end()) {
+            // ### breaks RVO on most compilers (but only on old-fashioned ones, so who cares?)
             T result(std::move(i->second));
             d->m.erase(i);
             return result;
         }
+#endif
         return T();
     }
 
@@ -1038,12 +1044,18 @@ public:
         // elements (the one to be removed can be skipped).
         detach();
 
+#ifdef __cpp_lib_node_extract
+        if (const auto node = d->m.extract(key))
+            return std::move(node.mapped());
+#else
         auto i = d->m.find(key);
         if (i != d->m.end()) {
+            // ### breaks RVO on most compilers (but only on old-fashioned ones, so who cares?)
             T result(std::move(i->second));
             d->m.erase(i);
             return result;
         }
+#endif
         return T();
     }