Work around compiler problems on MSVC 16.6.X

The compiler apparently doesn't resolve the emplace overload
correctly to the rvalue version, leading to infinite recursion
in the overload that takes a const Key & as argument.

Take a copy to make it explicit.

Change-Id: I22039d6ca1e2176c81e51b181c72f511dab662f7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qhash.h

@@ -1121,7 +1121,8 @@ public:
     template <typename ...Args>
     iterator emplace(const Key &key, Args &&... args)
     {
-        return emplace(Key(key), std::forward<Args>(args)...);
+        Key copy = key; // Needs to be explicit for MSVC 2019
+        return emplace(std::move(copy), std::forward<Args>(args)...);
     }
 
     template <typename ...Args>