[macro_metavars_in_unsafe]: recognize metavariables in tail expressions

[y21]

Fixes #13219

macro_metavars_in_unsafe keeps track of the current "expansion depth" (incremented/decremented when entering/leaving a macro span) to tell if an expression from the root context is contained within a macro (see the doc comment I added for a hopefully better explanation)

Before this PR, we didn't increment said expn_depth for unsafe blocks within macros, because we already do that in visit_stmt anyway, so it would work fine for statements, but that's not enough for tail expressions of an unsafe block.

So we now also increment it for macro unsafe blocks.
Also updated the comment for expn_depth while I'm at it because "This is not necessary for correctness" isn't correct now that I think about it

---

changelog: none

[rustbot]

r? @dswij

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[ojeda]

That was quick, thanks!

[dswij]

Thank you!

[dswij]

@bors r+

[bors]

📌 Commit 234a1d3 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit 234a1d3 with merge 37f98ff...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing 37f98ff to master...