Improve diagnostic-related lints: `untranslatable_diagnostic` & `diagnostic_outside_of_impl`
Summary:
- Made `untranslatable_diagnostic` point to problematic arguments instead of the function call
(I found this misleading while working on some `A-translation` PRs: my first impression was that
the methods themselves were not translation-aware and needed to be changed,
while in reality the problem was with the hardcoded strings passed as arguments).
- Made the shared pass of `untranslatable_diagnostic` & `diagnostic_outside_of_impl` more efficient.
`@rustbot` label D-imprecise-spans A-translation
47af700fe6