[libc++][Modules] Add missing includes and exports

[libc++][Modules] Add missing includes and exports

Several headers are missing includes for things they use.

type_traits.is_enum needs to export type_traits.integral_constant so that clients can access its `value` member without explicitly including __type_traits/integral_constant.h themselves.

Make `subrange_fwd` a peer submodule to `subrange` rather than a submodule of it, and have `subrange` export `subrange_fwd`. That will make it easier to programmatically generate modules for the private detail headers, and it will accomplish the same effect that __ranges/subrange.h will make subrange_kind visible.

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D150055