Rollup merge of #125210 - fmease:fix-up-some-diags, r=davidtwco

Cleanup: Fix up some diagnostics

Several diagnostics contained their error code inside their primary message which is no bueno.
This PR moves them out of the message and turns them into structured error codes.

Also fixes another occurrence of `->` after a selector in a Fluent message which is not correct. I've fixed two other instances of this issue in #104345 (2022) but didn't update all instances as I've noted here: rust-lang/rust#104345 (comment) (“the future is now!”).

Author: matthiaskrgr