I am pretty sure it comes from the debugger - which is from the MS Visual Studio toolkit! I think I do not need to recompile the entire thing with mingw only to find this confirmed: It would take me probably several days with all the related modules...
Anyway, I could imagine to either have some "switch" to shut up the debugger (including a trick how to apply it from inside the QtCreator), or some option in the QtCreator itself that filters these messages out.
Actually I did not only switch from an earlier version of QtCreator to now 4.7.1, but also from MSVC 2013 to 2015 at the same time, so the problem could come from the one or the other. (And you are right: chances are high that it is actually the MSVC 2015 debugger...)