Change source identifier type in ProString

The strings remember in which file they were created/assigned.

However, this used a non-counting reference to a ProFile, which could
become dangling. If a subsequent ProFile re-used the exact same address,
a string's source would be mis-identified, which would be fatal in
conjunction with discard_from().

Since we actually need only a unique id for comparison, let's use an
integer for that.

Task-number: QTBUG-62434
Started-by: Simon Hausmann <simon.hausmann@qt.io>
Change-Id: I395153afaf7c835d0119690ee7f4b915e6f90d4a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
(cherry picked from qtbase/190aa94be7f5e146bef44862b974d733755cec85)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>