Ada: Fix incorrect legality check in instantiation of child generic unit

Ada: Fix incorrect legality check in instantiation of child generic unit

The problem arises when the generic unit has a formal access type parameter,
because the manual resolution implemented in Find_Actual_Type does not pick
the correct entity for the designated type.  The fix replaces it with a bona
fide resolution and cleans up the associated code in the callers.

gcc/ada/
	PR ada/18453
	* sem_ch12.adb (Find_Actual_Type): Add Typ_Ref parameter and
	perform a standard resolution on it in the fallback case.
	Call Get_Instance_Of if the type is declared in a formal of
	the child unit.
	(Instantiate_Type.Validate_Access_Type_Instance): Adjust call
	to Find_Actual_Type.
	(Instantiate_Type.Validate_Array_Type_Instance): Likewise and
	streamline the check for matching component subtypes.

gcc/testsuite/
	* gnat.dg/specs/generic_inst9.ads: New test.
	* gnat.dg/specs/generic_inst9_pkg1.ads: New helper.
	* gnat.dg/specs/generic_inst9_pkg2.ads: Likewise.
	* gnat.dg/specs/generic_inst9_pkg2-g.ads: Likewise.