Add a tidy check that checks whether the fluent slugs only appear once

[mu001999]

As @Nilstrieb said in #121828 (comment):

Might make sense to have a tidy check that checks whether the fluent slugs only appear once in the source code and lint for that
there's a tidy check already for sorting

We can get the tidy check error:

tidy check
tidy error: /path/to/rust/compiler/rustc_const_eval/messages.ftl: message `const_eval_invalid_align` is not used
tidy error: /path/to/rust/compiler/rustc_lint/messages.ftl: message `lint_trivial_untranslatable_diag` is not used
tidy error: /path/to/rust/compiler/rustc_parse/messages.ftl: message `parse_invalid_literal_suffix` is not used
tidy error: /path/to/rust/compiler/rustc_infer/messages.ftl: message `infer_need_type_info_in_coroutine` is not used
tidy error: /path/to/rust/compiler/rustc_passes/messages.ftl: message `passes_expr_not_allowed_in_context` is not used
tidy error: /path/to/rust/compiler/rustc_passes/messages.ftl: message `passes_layout` is not used
tidy error: /path/to/rust/compiler/rustc_parse/messages.ftl: message `parse_not_supported` is not used

r? @Nilstrieb

[bors]

☔ The latest upstream changes (presumably #121859) made this pull request unmergeable. Please resolve the merge conflicts.

[Noratrieb]

Did a proper review of it, and I think the algorithm can be simplified. After that, happy to merge! I was surprised to see that the performance of it is still really good, not being slower than before.. I guess compute is just so cheap compared to (my quite fast!) disk :)

[Noratrieb]

Awesome, thanks!
@bors r+

[bors]

📌 Commit d88c7ff has been approved by Nilstrieb

It is now in the queue for this repository.

[mu001999]

Is there anything wrong? Why hasn't this PR been merged yet 🤔

[Noratrieb]

it's in the queue, weird.

[Nadrieril]

@bors rollup