qmake: use new QString::arg(QStringView) overload

Add ProString::toQStringView() to avoid creating QStrings just to pass
them to QString::arg() (single-arg; multiArg() does not, yet, accept
QStringViews).

I could have used the existing toQStringRef() function, but QStringRef
is a tad more complex to copy and quite a bit less future-proof.

Change-Id: I344c46f301768e844c487d36ce3e6cb276de8843
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
(cherry picked from qtbase/f5d8ad61a4c85a656a7332c43d0c42f5eaf43593)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>