ada: Enforce checks on access to interface type conversions

ada: Enforce checks on access to interface type conversions

The patch enforces checks on access to interface type conversions
internally generated by the frontend to displace the pointer to
a tagged type object (pointer named "this" in the C++ terminology)
from a dispatch table to a another dispatch table.

gcc/ada/ChangeLog:

	* exp_util.ads (Flag_Interface_Pointer_Displacement): New subprogram.
	* exp_util.adb (Flag_Interface_Pointer_Displacement): Ditto.
	* exp_attr.adb (Add_Implicit_Interface_Type_Conversion): Flag type
	conversions internally added to displace the pointer to the object.
	(Expand_N_Attribute_Reference): Ditto.
	* exp_ch4.adb (Displace_Allocator_Pointer): Ditto.
	* exp_ch6.adb (Expand_Simple_Function_Return): Ditto.
	(Make_Build_In_Place_Call_In_Allocator): Ditto.
	(Make_CPP_Constructor_Call_In_Allocator): Ditto.
	* exp_disp.adb (Expand_Interface_Actuals): Ditto.
	* exp_intr.adb (Expand_Dispatching_Constructor_Call): Ditto.
	* sem_ch6.adb (Analyze_Function_Return): Ditto.
	* sem_disp.adb (Propagate_Tag): Ditto.
	* sem_res.adb (Resolve_Actuals): Ditto.
	(Valid_Conversion): Rely on the new flag to handle the type conversion
	as a conversion added to displace the pointer to the object. Factorize
	code handling general and anonymous access types.
	* sem_type.adb (Interface_Present_In_Ancestor): For concurrent types
	add missing handling of class-wide types. Noticed working on this
	issue.
	* sinfo.ads (Is_Interface_Pointer_Displacement): Document this new flag.
	* gen_il-fields.ads (Is_Interface_Pointer_Displacement): New flag.
	* gen_il-gen-gen_nodes.adb (Is_Interface_Pointer_Displacement): New
	flag on N_Type_Conversion nodes.
	* gen_il-internals.adb (Image): Add Is_Interface_Pointer_Displacement
	flag image.