Fixes#64590
When we emit an 'unreachable' lint, we now add a note pointing at the
expression that actually causes the code to be unreachable (e.g.
`return`, `break`, `panic`).
This is especially useful when macros are involved, since a diverging
expression might be hidden inside of a macro invocation.
For the most part, the current code performs similarly, although it
differs in some particulars. I'll be nice to have these tests for
judging future changes, as well.
