c++: remove function/var concepts code

This patch removes vestigial Concepts TS code as discussed in
<https://gcc.gnu.org/pipermail/gcc-patches/2024-July/657937.html>.

In particular, it removes code related to function/variable concepts.
That includes variable_concept_p and function_concept_p, which then
cascades into removing DECL_DECLARED_CONCEPT_P etc.  So I think we
no longer need to say "standard concept" since there are no non-standard
ones anymore.

I've added two new errors saying that "variable/function concepts are
no longer supported".

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Don't call
	unpack_concept_check.  Add a concept_check_p assert.  Remove
	function_concept_p code.
	* constraint.cc (check_constraint_atom): Remove function concepts code.
	(unpack_concept_check): Remove.
	(get_concept_check_template): Remove Concepts TS code.
	(resolve_function_concept_overload): Remove.
	(resolve_function_concept_check): Remove.
	(resolve_concept_check): Remove Concepts TS code.
	(get_returned_expression): Remove.
	(get_variable_initializer): Remove.
	(get_concept_definition): Remove Concepts TS code.
	(normalize_concept_check): Likewise.
	(build_function_check): Remove.
	(build_variable_check): Remove.
	(build_standard_check): Use concept_definition_p instead of
	standard_concept_p.
	(build_concept_check): Remove variable_concept_p/function_concept_p
	code.
	(build_concept_id): Simplify.
	(build_type_constraint): Likewise.
	(placeholder_extract_concept_and_args): Likewise.
	(satisfy_nondeclaration_constraints): Likewise.
	(check_function_concept): Remove.
	(get_constraint_error_location): Remove Concepts TS code.
	* cp-tree.h (DECL_DECLARED_CONCEPT_P): Remove.
	(check_function_concept): Remove.
	(unpack_concept_check): Remove.
	(standard_concept_p): Remove.
	(variable_concept_p): Remove.
	(function_concept_p): Remove.
	(concept_definition_p): Simplify.
	(concept_check_p): Don't check for CALL_EXPR.
	* decl.cc (check_concept_refinement): Remove.
	(duplicate_decls): Remove check_concept_refinement code.
	(is_concept_var): Remove.
	(cp_finish_decl): Remove is_concept_var.
	(check_concept_fn): Remove.
	(grokfndecl): Give an error about function concepts not being supported
	anymore.  Remove unused code.
	(grokvardecl): Give an error about variable concepts not being
	supported anymore.
	(finish_function): Remove DECL_DECLARED_CONCEPT_P code.
	* decl2.cc (min_vis_expr_r): Use concept_definition_p instead of
	standard_concept_p.
	(maybe_instantiate_decl): Remove DECL_DECLARED_CONCEPT_P check.
	(mark_used): Likewise.
	* error.cc (dump_simple_decl): Use concept_definition_p instead of
	standard_concept_p.
	(dump_function_decl): Remove DECL_DECLARED_CONCEPT_P code.
	(print_concept_check_info): Don't call unpack_concept_check.  Simplify.
	* mangle.cc (write_type_constraint): Likewise.
	* parser.cc (cp_parser_nested_name_specifier_opt): Remove
	function_concept_p code.  Only check concept_definition_p, not
	variable_concept_p/standard_concept_p.
	(add_debug_begin_stmt): Remove DECL_DECLARED_CONCEPT_P code.
	(cp_parser_template_declaration_after_parameters): Remove a stale
	comment.
	* pt.cc (check_explicit_specialization): Remove
	DECL_DECLARED_CONCEPT_P code.
	(process_partial_specialization): Remove variable_concept_p code.
	(lookup_template_variable): Likewise.
	(tsubst_expr) <case CALL_EXPR>: Remove Concepts TS code and simplify.
	(do_decl_instantiation): Remove DECL_DECLARED_CONCEPT_P code.
	(instantiate_decl): Likewise.
	(placeholder_type_constraint_dependent_p): Don't call
	unpack_concept_check.  Add a concept_check_p assert.
	(convert_generic_types_to_packs): Likewise.
	* semantics.cc (finish_call_expr): Remove Concepts TS code and simplify.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/decl-diagnose.C: Adjust dg-error.
	* g++.dg/concepts/fn-concept2.C: Likewise.
	* g++.dg/concepts/pr71128.C: Likewise.
	* g++.dg/concepts/var-concept6.C: Likewise.
	* g++.dg/cpp2a/concepts.C: Likewise.