[libc++] Keep unary_function and binary_function in C++17 for one more release

In LLVM 15, we added the deprecation markup for unary_function and
binary_function for >= C++11, and we also removed it for >= C++17.
While this is in accordance with the Standard, it's also a bit quick
for our users, since there was no release in which the classes were
marked as deprecated before their removal.

We noticed widespread breakage due to this, and after months of trying
to fix downstream failures, I am coming to the conclusion that users
will be better served if we give them one release where unary_function
is deprecated but still provided even in >= C++17.

Differential Revision: https://reviews.llvm.org/D134473