We've said that we'll remove `std::function` from C++03 in LLVM 16, so we might as well do it now before we forget.
Reviewed By: ldionne, #libc, Mordante
Spies: jloser, Mordante, libcxx-commits
Differential Revision: https://reviews.llvm.org/D135868
`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