Fill out MIR section on constants and constant promotion

[jyn514]

https://rustc-dev-guide.rust-lang.org/mir/index.html#representing-constants just says 'To be written'

cc @oli-obk @RalfJung I think this is your area of expertise?

[RalfJung]

Yeah, I think so.
But also we plan some fundamental changes there with rust-lang/rust#72396 which makes everything a lot cleaner.