c++: Unify REQUIRES_EXPR evaluation / diagnostic routines

c++: Unify REQUIRES_EXPR evaluation / diagnostic routines

This folds the diagnose_requires_expr routines into the corresponding
tsubst_requires_expr ones.  This is achieved by making the latter
routines take a sat_info instead of a subst_info, and assigning the
appropriate meanings to the flags sat_info::noisy and
sat_info::diagnose_unsatisfaction_p during tsubst_requires_expr:
info.noisy() controls whether to diagnose invalid types and expressions
inside the requirements, and info.diagnose_unsatisfaction_p() controls
whether to additionally diagnose why the requires-expression evaluates
to false.

gcc/cp/ChangeLog:

	* constraint.cc (struct sat_info): Document the different
	meanings of noisy() and diagnose_unsatisfaction_p() during
	satisfaction and requires-expression evaluation.
	(tsubst_valid_expression_requirement): Take a sat_info instead
	of a subst_info.  Perform the substitution quietly first.  Fold
	in error-replaying code from diagnose_valid_expression.
	(tsubst_simple_requirement): Take a sat_info instead of a
	subst_info.
	(tsubst_type_requirement_1): New.  Fold in error-replaying code
	from diagnose_valid_type.
	(tsubst_type_requirement): Use the above.  Take a sat_info
	instead of a subst_info.
	(tsubst_compound_requirement): Likewise.  Fold in
	error-replaying code from diagnose_compound_requirement.
	(tsubst_nested_requirement): Take a sat_info instead of a
	subst_info.  Fold in error-replaying code from
	diagnose_nested_requirement.
	(tsubst_requirement): Take a sat_info instead of a subst_info.
	(tsubst_requires_expr): Split into two versions, one that takes
	a sat_info argument and another that takes a complain and
	in_decl argument.  Remove outdated documentation.  Document the
	effects of the sat_info argument.  Don't short-circuit
	processing of requirements when diagnosing unsatisfaction,
	mirroring diagnose_requires_expr.
	(satisfy_nondeclaration_constraint) <case REQUIRES_EXPR>: Remove
	assert, and se the three-parameter version of tsubst_requires_expr.
	(diagnose_trait_expr): Make static.  Take a template argument
	vector instead of a parameter mapping.
	(diagnose_valid_expression): Remove.
	(diagnose_valid_type): Remove.
	(diagnose_simple_requirement): Remove.
	(diagnose_compound_requirement): Remove.
	(diagnose_type_requirement): Remove.
	(diagnose_nested_requirement): Remove.
	(diagnose_requirement): Remove.
	(diagnose_requires_expr): Remove.
	(diagnose_atomic_constraint): Take a sat_info instead of a
	subst_info.  Adjust call to diagnose_trait_expr.  Call
	tsubst_requires_expr instead of diagnose_requires_expr.
	(diagnose_constraints): Remove special casing of REQUIRES_EXPR
	and just always call constraint_satisfaction_value.