This mostly copys the `<experimental/functional>` stuff and updates the code to current libc++ style.
Reviewed By: ldionne, #libc
Spies: nlopes, adamdebreceni, arichardson, libcxx-commits, mgorny
Differential Revision: https://reviews.llvm.org/D121074
When some headers are not available because we removed features like
localization or threads, the compiler should not try to include these
headers when building modules. To avoid that from happening, add a
requires-declaration that is never satisfied when the configuration
in use doesn't support a header.
rdar://93777687
Differential Revision: https://reviews.llvm.org/D127127