Don't look for safety comments in doc tests

[y21]

Fixes #12048.

What happened in the linked issue is that the lint checks for lines that start with // and have SAFETY: somewhere in it above the function item.
This works for regular comments, but when the // is the start of a doc comment (e.g. /// // SAFETY: ...) and it's part of a doc test (i.e. within ```), we probably shouldn't lint that, since the user most likely meant to refer to a different node than the one currently being checked. For example in the linked issue, the safety comment refers to unsafe { *five_pointer }, but the lint believes it's part of the function item.

We also can't really easily test whether the // SAFETY: comment within a doc comment is necessary or not, since I think that would require creating a new compiler session to re-parse the contents of the doc comment. We already do this for one of the doc markdown lints, to look for a main function in doc tests, but I don't know how to feel about doing that in more places, so probably best to just ignore them?

changelog: [unnecessary_safety_comment]: don't look for safety comments in doc tests

[rustbot]

r? @Alexendoo

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[Alexendoo]

👍

@bors r+

[bors]

📌 Commit ef35e82 has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit ef35e82 with merge d3dfecb...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing d3dfecb to master...