Fix internal error on function call returning extension of limited interface

This is a regression present on the mainline and 13 branch, in the form of a
series of internal errors (3) on a function call returning the extension of
a limited interface.

This is only a partial fix for the first two assertion failures; the third
one is the most problematic and will be dealt with separately.

The first issue is in Instantiate_Type, where we use Base_Type in a specific
case to compute the ancestor of a derived type, which will later trigger the
assertion on line 16960 of sem_ch3.adb since Parent_Base and Generic_Actual
are the same node.  This is changed to use Etype like in other cases around.

The second issue is an unprotected use of Designated_Type on type T in
Analyze_Explicit_Dereference, while another use in an equivalent context
is guarded by Is_Access_Type a few lines above.

gcc/ada
	PR ada/112781
	* sem_ch12.adb (Instantiate_Type): Use Etype instead of Base_Type
	consistently to retrieve the ancestor for a derived type.
	* sem_ch4.adb (Analyze_Explicit_Dereference): Test Is_Access_Type
	consistently before accessing Designated_Type.