[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",
Differential Revision: https://reviews.llvm.org/D101293