exhaustiveness: Explain why a given pattern is considered unreachable
This PR tells the user why a given pattern is considered unreachable. I reused the intersection information we were already computing; even though it's incomplete I convinced myself that it is sufficient to always get a set of patterns that cover the unreachable one.
I'm not a fan of the diagnostic messages I came up with, I'm open to suggestions.
Fixes https://github.com/rust-lang/rust/issues/127870. This is also the other one of the two diagnostic improvements I wanted to do before https://github.com/rust-lang/rust/pull/122792.
Note: the first commit is an unrelated drive-by tweak.
r? `@compiler-errors`
355efacf0d