Unused Fluent messages lint

[davidtwco]

Inspired by #110597 - we should add a check for unused Fluent messages.

[est31]

@rustbot claim

Basically, one would need to: traverse the directories, parse the ftl files, and for each ftl file found, determine if the fluent messages contained are either used internally, or used somewhere else inside the compiler. This can work on a simple token/word level, as using syn in tidy isn't really an option, as the compiler uses unstable syntax.

[tgross35]

Something like this was noticed in #127935 - we had two labels called .note and only one was printing.

[est31]

Unassigning myself as I didn't find the time for it.

[tgross35]

An easier way might be to accept a BOOTSTRAP_FLUENT_OUTPUT_PATH env and, if set, the proc macros append the used keys with spans to that file after completing (just a simple JSON template or something). Then tidy can just diff what's there against the fluent files.

Possibly less robust I guess, but bootstrap has pretty good control over the environment. And tracked_env / tracked_path help.

[jieyouxu]

cc #132181

[clubby789]

iirc I did work on this previously by just making the fluent constants pub(crate). I dropped it as there were a couple of diagnostic messages used cross-crate that I didn't want to rework, but at the time there were no violations