[Ada] Introduce hardbool Machine_Attribute for Ada

[Ada] Introduce hardbool Machine_Attribute for Ada

Implement and document hardened booleans, from nonstandard boolean types
with representation clauses to the extra validity checking performed on
boolean types annotated with the "hardbool" Machine_Attribute pragma.

gcc/ada/

	* doc/gnat_rm/security_hardening_features.rst (Hardened
	Booleans): New.
	* exp_util.adb (Adjust_Condition): Perform validity checking on
	hardbool-annotated types even with -gnatVT.
	* gnat_rm.texi: Regenerate.
	* gcc-interface/utils.cc (gnat_internal_attribute_table): Ignore
	hardbool.