[Bug libstdc++/99979] condition_variable_any has wrong behavior if Lock::lock() throws

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99979

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The _Unlock type was introduced for PR libstdc++/50862 (and then modified
slightly by PR libstdc++/53830).