[libc++] Remove std::function in C++03

`std::function` has been deprecated for a few releases now. Remove it with an option to opt-back-in with a note that this option will be removed in LLVM 16.

Reviewed By: ldionne, #libc

Spies: #libc_vendors, EricWF, jloser, libcxx-commits

Differential Revision: https://reviews.llvm.org/D127908