Fix SAFETY comment tag casing in undocumented_unsafe_blocks

[wooster0]

This changes the lint introduced in #7748 to suggest adding a SAFETY comment instead of a Safety comment.

Searching for // Safety: in rust-lang/rust yields 67 results while // SAFETY: yields 1072.
I think it's safe to say that this comment tag is written in upper case, just like TODO, FIXME and so on are. As such I would expect this lint to follow the official convention as well.

Note that I intentionally introduced some casing diversity in tests/ui/undocumented_unsafe_blocks.rs to test more cases than just Safety:.

changelog: Capitalize SAFETY comment in [undocumented_unsafe_blocks]

[rust-highfive]

r? @giraffate

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[xFrednet]

I like standardizing the capitalization. 👍 You might also want to take a look at the missing_safety_doc lint. That one lints missing safety heading in doc comments. 🙃

[wooster0]

@xFrednet I think in that case /// # Safety is fine as-is because it denotes a markdown section like e.g. /// # Examples does (which is not uppercased either). I believe that and this comment tag // SAFETY: which is directly followed by the explanation about why it is safe are two slightly different things.

[giraffate]

@bors r+

Thanks!

[bors]

📌 Commit eba4413 has been approved by giraffate

[bors]

⌛ Testing commit eba4413 with merge 7905130...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing 7905130 to master...