ada: Fix error message about limited extensions

ada: Fix error message about limited extensions

Consider the following package:

    package P is
       type T1 is tagged limited null record;

       type T2 is new T1 with private;
    private

       type T2 is limited new T1 with null record;
    end P;

It should be rejected because of ARM 7.3 (10.1/3). Before this patch,
GNAT did reject it, with the following error message:

    full view of non-limited extension cannot be limited

This message is not right because the partial view of T2 here *is*
limited even if it doesn't have an explicit limited keyword in its
declaration. This patch changes the error message to something that's a
better match for ARM 7.3 (10.1/3).

This patch also tweaks another related error message and substitutes
a mention of ARM 7.3 (10.1/3) for the Ada Issue it originated from in a
comment.

gcc/ada/ChangeLog:

	* sem_ch3.adb (Process_Full_View): Fix error message.