[libc++] Move <__sso_allocator> out of include/ into src/. NFCI.

[libc++] Move <__sso_allocator> out of include/ into src/. NFCI.

This allocator is not intended for libc++'s users to use;
it's strictly an implementation detail of `src/locale.cpp`.
So, move 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", etc.

Differential Revision: https://reviews.llvm.org/D101293