[Bug libstdc++/92143] std::pmr::polymorphic_allocator throws bad_alloc on macOS

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92143

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far, but I'll backport it too.