[Ada] Improve integration of strub with type systems

[Ada] Improve integration of strub with type systems

gcc/ada/

	* strub.adb, strub.ads: New files.
	* exp_attr.adb (Access_Cases): Copy strub mode to subprogram type.
	* exp_disp.adb (Expand_Dispatching_Call): Likewise.
	* freeze.adb (Check_Inherited_Conditions): Check that strub modes
	match overridden subprograms and interfaces.
	(Freeze_All): Renaming declarations too.
	* sem_attr.adb (Resolve_Attribute): Reject 'Access to
	strub-annotated data object.
	* sem_ch3.adb (Derive_Subprogram): Copy strub mode to
	inherited subprogram.
	* sem_prag.adb (Analyze_Pragma): Propagate Strub Machine_Attribute
	from access-to-subprogram to subprogram type when required,
	but not from access-to-data to data type.  Mark the entity that
	got the pragma as having a gigi rep item.
	* sem_res.adb (Resolve): Reject implicit conversions that
	would change strub modes.
	(Resolve_Type_Conversions): Reject checked conversions
	between incompatible strub modes.
	* doc/gnat_rm/security_hardening_features.rst: Update.
	* gnat_rm.texi: Regenerate.
	* libgnat/a-except.ads (Raise_Exception): Revert strub-callable
	annotation in public subprogram.
	* libgnat/s-arit128.ads (Multiply_With_Ovflo_Check128): Likewise.
	* libgnat/s-arit64.ads (Multiply_With_Ovflo_Check64): Likewise.
	* libgnat/s-secsta.ads (SS_Allocate): Likewise.
	(SS_Mark, SS_Release): Likewise.
	* gcc-interface/Make-lang.in (GNAT_ADA_OBJS): Add ada/strub.o.