[libc] Add a TableGen based header generator.

Summary:
* The Python header generator has been removed.
* Docs giving a highlevel overview of the header gen scheme have been
  added.

Reviewers: phosek, abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D70197