qmake: make IoUtils::fileType() return more expressive result

it now does not see anything except regular files and directories any
more. that's not expected to be a problem, given the function's scope.

Change-Id: I53063ad8cacb3afe5cc1baf6d6d5feba3465e74f
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtcreator/cf82f210804151452fce3cddb3cb2793dab976eb)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>