Extend QQmlElement with empty placement delete operator

The signature is based on [1] and [2], with an empty implementation to match the
default behavior.

This mutes the following warning when building with MSVC 2015 & 2017:
warning: C4291: 'void *operator new(::size_t,void *) throw()': no matching operator delete found; memory will not be freed if initialization throws an exception

[1] https://en.cppreference.com/w/cpp/memory/new/operator_delete (#13)
[2] http://www.cplusplus.com/reference/new/operator%20delete/ (#3)

Task-number: QTBUG-71024
Change-Id: I32f80a902672d9af27960a185a1b0c91798806c5
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/qml/qml/qqmlprivate.h

@@ -109,6 +109,10 @@ namespace QQmlPrivate
             // the size that was allocated.
             ::operator delete (ptr);
         }
+        static void operator delete(void *, void *) {
+            // Deliberately empty placement delete operator.
+            // Silences MSVC warning C4291: no matching operator delete found
+        }
     };
 
     template<typename T>