Auto merge of #119443 - Urgau:move-around-builtin-diag-context, r=Nilstrieb

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 #119425 (comment)

r? `@Nilstrieb`

Author: bors