[Bug libstdc++/58159] unique_ptr::reset should have debug assertion for "self-reset"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58159

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
What if the deleter doesn't actually destroy the object, and doing self-reset
is used as a crazy way to trigger the deleter to do something with the pointer,
but not to alter the value of the pointer?

If that's valid then we should only do this for default_delete specializations.