builtins: Fix up strspn/strcspn folding [PR119219]

builtins: Fix up strspn/strcspn folding [PR119219]

The PR119204 r15-7955 fix caused some regressions.
The problem is that the fold_builtin* APIs document that expr is
either a CALL_EXPR of the call or NULL, so using TREE_TYPE (expr)
can crash e.g. during constexpr evaluation etc.

As can be seen in the surrounding patch, for the neighbouring builtins
(both modf and strpbrk) fold_builtin_2 passes down type, which is the
result type, TREE_TYPE (TREE_TYPE (fndecl)) and those builtins use it
to build the return value, while strspn was always building size_type_node
and strcspn had this change from that to TREE_TYPE (expr).
The patch passes type to these two and uses it there as well.

The patch keeps passing expr because it is used in the
check_nul_terminated_array calls done for both strspn and strcspn,
those calls clearly can deal with NULL expr but prefer if it is non-NULL
for some warning.

2025-03-12  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/119204
	PR middle-end/119219
	* builtins.cc (fold_builtin_2): Pass type as another argument
	to fold_builtin_strspn and fold_builtin_strcspn.
	(fold_builtin_strspn): Add type argument, use it instead of
	size_type_node.
	(fold_builtin_strcspn): Add type argument, use it instead of
	TREE_TYPE (expr).