[Ada] Fix bogus visibility error with partially parameterized formal package

[Ada] Fix bogus visibility error with partially parameterized formal package

The problem comes from the special instantiation (abbreviated instantiation
in GNAT parlance) done to check conformance between a formal package and its
corresponding actual in a generic instantiation: the compiler instantiates
the formal package, in the context of the generic instantiation, so that it
can check the conformance of the actual with the result.

More precisely, it occurs with formal packages that are only partially
parameterized, i.e. that have at least one parameter association and an
(others => <>) choice. In this case, RM 12.7(10/2) says that the visible
part of the formal package contains a copy of the formal parameters that
are not explicitly associated.

The analysis of these copies for the abbreviated instantiation is not done
in the correct context when the generic unit is a child generic unit.

gcc/ada/

	* sem_ch12.ads (Is_Abbreviated_Instance): Declare.
	* sem_ch12.adb (Check_Abbreviated_Instance): Declare.
	(Requires_Conformance_Checking): Declare.
	(Analyze_Association.Process_Default): Fix subtype of parameter.
	(Analyze_Formal_Object_Declaration): Check whether it is in the
	visible part of abbreviated instance.
	(Analyze_Formal_Subprogram_Declaration): Likewise.
	(Analyze_Formal_Type_Declaration): Likewise.
	(Analyze_Package_Instantiation): Do not check for a generic child
	unit in the case of an abbreviated instance.
	(Check_Abbreviated_Instance): New procedure.
	(Check_Formal_Packages): Tidy up.
	(Copy_Generic_Elist): Fix comment.
	(Instantiate_Formal_Package): Tidy up.  If the generic unit is a
	child unit, copy the qualified name onto the abbreviated instance.
	(Is_Abbreviated_Instance): New function.
	(Collect_Previous_Instances): Call Is_Abbreviated_Instance.
	(Requires_Conformance_Checking): New function.
	* sem_ch7.adb (Analyze_Package_Specification): Do not install the
	private declarations of the parent for an abbreviated instance.