Add a new warning message that is generated if an instance of an abstract class is created. This message is disabled by default, but can be switched on by using {$warn 4122 on} or {$warn 4122 error}.

Please note that this warning won't be triggered if an instance of that class is created using a class variable of that class type as the compiler can not know the type contained in the variable at compile time (see also the added test).

+ msg/errore.msg: added disabled message which informs about the instantiation of an abstract class
* pexpr.pas, do_member_read: generate the message if we have a constructor call for an abstract class using a loadvmtaddrnode (thus the type name is used and not a class variable)

* msg{idx,txt}.inc: updated

+ added test

git-svn-id: trunk@28127 -