Move around the code responsible for decorating builtin diagnostics
This PR move the code responsible for decorating builtin diagnostics into a separate sub-module for ease of use and readability.
While my original intention was to also move the check-cfg unexpected logic in their own function I changed my mind after moving the match altogether. I can move those if desired.
Fixes https://github.com/rust-lang/rust/pull/119425#discussion_r1438446596
r? `@Nilstrieb`