[Bug libstdc++/106839] Library traits need to handle array of incomplete type with unknown bound

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106839

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
As noted in PR 106838 the library needs to guard use of
__has_virtual_destructor so it's only used with non-union class types. This
applies to trunk as well.