Rollup merge of rust-lang#65265 - GuillaumeGomez:cleanup-librustc_mir-err-codes, r=Mark-Simulacrum

Cleanup librustc mir err codes

Three things are done in this PR:

 * Sort error codes
 * Uncomment an error code long error explanation (they should **never** be commented)
 * Unify explanations

Author: tmandry