[Bug libstdc++/91871] iterator_to_const_iterator() in testsuite_hooks.h causes valid -Wreturn-stack-address warnings with LLVM

[brooks at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91871

--- Comment #7 from Brooks Moses <brooks at gcc dot gnu.org> ---
Thanks, Jonathan!  I've confirmed that this does indeed fix the warning with
Clang trunk, and the test passes again.