As far as I can tell, this This allocator is not intended for libc++'s users to use;
it's strictly an implementation detail of `src/locale.cpp`.
So, Somove it to the `src/include/` directory.
Drive-by const-qualify its comparison operators.
For consistency with `__hidden_allocator` (defined in `src/thread.cpp`),
do *not* remove it from "libcxx/lib/libc++unexp.exp",
"libcxx/utils/symcheck-blacklists/linux_blacklist.txt", move it to the `src/include/` directoryetc.