Rollup merge of #86533 - inquisitivecrystal:lower-case-error-explain, r=petrochenkov

Rollup merge of #86533 - inquisitivecrystal:lower-case-error-explain, r=petrochenkov

Support lowercase error codes in `--explain`

This enables `rustc --explain` to accept a lowercase error code. Thus, for instance, `rustc --explain e0573` would be valid after this change, where before a user would have needed to do `rustc --explain E0573`. Although the upper case form of an error code is canonical, the user may prefer the easier-to-type lowercase form, and there's nothing to be gained by forcing them to type the upper case version.

Resolves #86518.