libstdc++: Implement DR 526 on [forward_]list remove_if/unique [PR 91620]

libstdc++: Implement DR 526 on [forward_]list remove_if/unique [PR 91620]

Respect DR 526 in implementation of std::[forward_]list remove/remove_if/unique.
[forward_]list::remove was already implementing it but the implementation has
been modified to generalize the following pattern. All nodes to remove are
collected in an intermediate [forward_]list which purpose is just to be
detroyed once out of scope.

libstdc++-v3/ChangeLog:

	PR libstdc++/91620
	* include/bits/forward_list.tcc (forward_list<>::remove): Collect nodes
	to destroy in an intermediate forward_list.
	(forward_list<>::remove_if, forward_list<>::unique): Likewise.
	* include/bits/list.tcc (list<>::remove, list<>::unique): Likewise.
	(list<>::remove_if): Likewise.
	* include/debug/forward_list (forward_list<>::_M_erase_after): Remove.
	(forward_list<>::erase_after): Adapt.
	(forward_list<>::remove, forward_list<>::remove_if): Collect nodes to
	destroy in an intermediate forward_list.
	(forward_list<>::unique): Likewise.
	* include/debug/list (list<>::remove, list<>::unique): Likewise.
	(list<>::remove_if): Likewise.
	* testsuite/23_containers/forward_list/operations/91620.cc: New test.
	* testsuite/23_containers/list/operations/91620.cc: New test.