c++: Simplify constraint normalization routines

c++: Simplify constraint normalization routines

Many of the high-level constraint normalization routines allow the
caller to supply the initial template arguments for normalization, but
in practice all of the callers supply something equivalent to the
identity mapping(*).

This patch hard-codes this prevalent choice of initial template
arguments by making get_normalized_constraints always pass NULL_TREE as
the args to normalize_expression.  This admits some simplifications in
the high-level routines, such as removing their 'args' parameter and
consolidating the two versions of normalize_constraint_expression.

(*): In particular, a set of generic template arguments or NULL_TREE.
In the case of the two-parm version of normalize_constraint_expression,
we were suspiciously using the template arguments of a concept-id when
normalizing the concept-id as a constraint-expression.

gcc/cp/ChangeLog:

	* constraint.cc (get_normalized_constraints): Remove 'args'
	parameter.  Pass NULL_TREE as the initial template arguments to
	normalize_expression.
	(get_normalized_constraints_from_info): Remove 'args' parameter
	and adjust the call to get_normalized_constraints.
	(get_normalized_constraints_from_decl): Remove 'args' local
	variable and adjust call to get_normalized_constraints_from_info.
	(normalize_concept_definition): Remove 'args' local variable
	and adjust call to get_normalized_constraints.
	(normalize_constraint_expression): Remove the two-parameter
	overload.  Remove 'args' parameter from the three-parameter
	overload and update function comment accordingly.  Remove
	default argument from 'diag' parameter.  Adjust call to
	get_normalized_constraints.
	(finish_nested_requirement): Adjust call to
	normalize_constraint_expression.
	(strictly_subsumes): Remove 'args' parameter.  Adjust call to
	get_normalized_constraints_from_info.
	(weakly_subsumes): Likewise.
	* cp-tree.h (strictly_subsumes): Remove 'args' parameter.
	(weakly_subsumes): Likewise.
	* pt.c (process_partial_specialization): Adjust call to
	strictly_subsumes.
	(is_compatible_template_arg): Adjust call to weakly_subsumes.