[Bug libstdc++/61645] forward_list::splice_after shall not throw exceptions

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61645

--- Comment #2 from lh_mouse <lh_mouse at 126 dot com> ---
(In reply to Daniel Krügler from comment #1)
> "Throws: Nothing" does not impose the requirement onto implementations that
> such a function is required to be marked equivalent to noexcept(true), so
> this looks more like an enhancement request to me - but let's wait what
> Jonathan or Pablo have to say.

Replace forward_list with list, splice_after with splice and before_begin with
begin, all three static_assert's hold.