libstdc++: Make std::erase for linked lists convert to bool

libstdc++: Make std::erase for linked lists convert to bool

LWG 4135 (approved in Wrocław, November 2024) fixes the lambda
expressions used by std::erase for std::list and std::forward_list.
Previously they attempted to copy something that isn't required to be
copyable. Instead they should convert it to bool right away.

The issue resolution also changes the lambda's parameter to be const, so
that it can't modify the elements while comparing them.

libstdc++-v3/ChangeLog:

	* include/std/forward_list (erase): Change lambda to have
	explicit return type and const parameter type.
	* include/std/list (erase): Likewise.
	* testsuite/23_containers/forward_list/erasure.cc: Check lambda
	is correct.
	* testsuite/23_containers/list/erasure.cc: Likewise.

Reviewed-by: Patrick Palka <ppalka@redhat.com>