[Ada] Implement predicate checks on qualified expressions (AI12-0100)

[Ada] Implement predicate checks on qualified expressions (AI12-0100)

2020-06-08  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* checks.adb (Apply_Predicate_Check): Refine test for being in a
	subprogram body to account for no Corresponding_Body case,
	avoiding blowups arising due to other changes here.
	* exp_ch4.adb (Expand_N_Qualified_Expression): Apply predicate
	checks, if any, after constraint checks are applied.
	* sem_eval.ads (Check_Expression_Against_Static_Predicate): Add
	Check_Failure_Is_Error formal for conditionalizing warning vs.
	error messages.
	* sem_eval.adb (Check_Expression_Against_Static_Predicate):
	Issue an error message rather than a warning when the new
	Check_Failure_Is_Error formal is True. In the nonstatic or
	Dynamic_Predicate case where the predicate is known to fail,
	emit the check to ensure that folded cases get checks applied.
	* sem_res.adb (Resolve_Qualified_Expression): Call
	Check_Expression_Against_Static_Predicate, passing True for
	Check_Failure_Is_Error, to ensure we reject static predicate
	violations. Remove code that was conditionally calling
	Apply_Predicate_Check, which is no longer needed, and that check
	procedure shouldn't be called from a resolution routine in any
	case. Also remove associated comment about preventing infinite
	recursion and consistency with Resolve_Type_Conversion, since
	that handling was already similarly removed from
	Resolve_Type_Convesion at some point.
	(Resolve_Type_Conversion): Add passing of True for
	Check_Failure_Is_Error parameter on call to
	Check_Expression_Against_Static_Predicate, to ensure that static
	conversion cases that violate a predicate are rejected as
	errors.