[clang] Move documentation about -verify from a header to public docs (PR #73694)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Would it make sense to document this more publicly? While it's designed to be an internal tool, it's really useful for people who want to make sure their library produces high quality diagnostics (e.g. `nodiscard`, `static_assert`s etc.). I'm sure you are aware that libc++ uses `-verify` too.

Because `-verify` is a `-cc1` option and not a driver-level option, it's not really user-facing despite being usable (and used!) outside of Clang. So I think the internals manual is the most appropriate place for the details; it's kind of akin to an internal-use pragma or intrinsic.

https://github.com/llvm/llvm-project/pull/73694