Different buildconfigurationfactories will always return different
BuildInfo, as the factories will turn the information into a different
set of objects (even if the information those objects are created from
are identical).

Each factory will always produce the same (sub-)type of BuildInfo, as
they need that information to proceed. So using the factory-address
as a kind of type-identifier is save.

Change-Id: Ia30355c26e045d453c0c2b75da893a4bac38b048
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>