c++: Implement __builtin_structured_binding_size trait

c++: Implement __builtin_structured_binding_size trait

clang++ apparently added a SFINAE-friendly __builtin_structured_binding_size
trait to return the structured binding size (or error if not in SFINAE
contexts if a type doesn't have a structured binding size).

The expansion statement patch already anticipated this through adding
complain argument to cp_finish_decomp.

The following patch implements it.

2025-08-15  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* doc/extend.texi (Type Traits): Document
	__builtin_structured_binding_size.
gcc/cp/
	* cp-trait.def (STRUCTURED_BINDING_SIZE): New unary trait.
	* cp-tree.h (finish_structured_binding_size): Declare.
	* semantics.cc (trait_expr_value): Handle
	CPTK_STRUCTURED_BINDING_SIZE.
	(finish_structured_binding_size): New function.
	(finish_trait_expr): Handle CPTK_RANK and CPTK_TYPE_ORDER
	in the switch instead of just doing break; for those and
	ifs at the end to handle them.  Handle CPTK_STRUCTURED_BINDING_SIZE.
	* pt.cc (tsubst_expr): Likewise.
	* constraint.cc (diagnose_trait_expr): Likewise.
	* decl.cc (get_tuple_size): Use mce_true for maybe_const_value.
	(cp_decomp_size): Diagnose incomplete types not just if
	processing_template_decl, and use error_at instead of pedwarn.
	If btype is NULL, just return 0 instead of diagnosing an error.
gcc/testsuite/
	* g++.dg/cpp26/expansion-stmt15.C: Expect different diagnostics
	for zero size destructuring expansion statement.
	* g++.dg/ext/builtin-structured-binding-size1.C: New test.
	* g++.dg/ext/builtin-structured-binding-size2.C: New test.
	* g++.dg/ext/builtin-structured-binding-size3.C: New test.
	* g++.dg/ext/builtin-structured-binding-size4.C: New test.