[Bug libstdc++/59807] mutex misses destructor if non-function call initialization is used

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59807

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In other words, we already have all the machinery in place to handle such
cases, it just needs to be used for the target.