Remove P<> from visit_s in ast MutVisitor

[maxcabrajac]

On #128974, one divergence I found between versions of the trait were P<> wrappers on some types on the mutable one.
Some are really easy to remove, while others are not.
This PR tries to remove them.

r? @petrochenkov

related to #128974 & #127615

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[petrochenkov]

(The PR was marked as a draft, so I assumed you want to try removing the rest of P<>s.)

[maxcabrajac]

Yup, that was the idea... But I'm starting to think this might not be a good one. From the Vistor's perspective it surely is, but when I looked at how rustc_expand heavily uses substitution on this P<>s it seems not to be great to remove them.

I'll look deeper into it on the weekend and keep you up to date!

If I end up finding a way to remove them from the visits then its best that we do another perf test.

[maxcabrajac]

For now, I removed P<>s from all visits, but this just pushed the problem into rustc_expand. What I see as the problem now is calling .into_inner() on the P<>s we get from .make_*() methods. A better approach is to change them to return T instead of P<T>. This would possibly remove some memory allocations, which I see as a good thing!

[bors]

☔ The latest upstream changes (presumably #131284) made this pull request unmergeable. Please resolve the merge conflicts.