[Bug libstdc++/103853] std::forward_list::merge should check if __list != this

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103853

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. this is https://cplusplus.github.io/LWG/issue3088

Currently the standard actually requires that x.merge(x) does x.clear() which
is obviously not what we want.