qmake: fix hypothetical raw data leaks relating to qt i/o classes

technically, we should not rely on the i/o classes not storing the
strings beyond the instantiated object's life time.

Change-Id: I0990769b3cf86860184869036c096c531160e9be
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtbase/702be65532263bd52ad0b67235c112083120699e)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>