Use AttrVec for Arm, FieldDef, and Variant

[JohnTitor]

Uses AttrVec for Arm, FieldDef, and Variant, i.e., where the size of the vector can be empty often.
Skips Crate and Item because I think they may have the attributes on common cases and need more work outside of rustc_ast (e.g. rustc_expand needs a lot of tweaks). But if it's reasonable to change, I'm happy to do so.

Fixes #77662

[rust-highfive]

Some changes occurred in src/tools/rustfmt.

cc @calebcartwright

[rust-highfive]

r? @davidtwco

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[calebcartwright]

rustfmt changes are good so providing approval accordingly

fwiw, the other changes lgtm to me as well and agree with thinking that attrs would be more common on Item and Crate

[davidtwco]

Looks good to me, seems fairly likely to be an perf improvement, but I'll do a perf run anyway to double check. r=me when perf results are good.

[davidtwco]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 4f8e0eb with merge a6b8035f4e90621eeba98861987b82318abcb25c...

[bors]

☀️ Try build successful - checks-actions
Build commit: a6b8035f4e90621eeba98861987b82318abcb25c (a6b8035f4e90621eeba98861987b82318abcb25c)

[rust-timer]

Queued a6b8035f4e90621eeba98861987b82318abcb25c with parent 0ef2b4a, future comparison URL.

[rust-timer]

Finished benchmarking try commit (a6b8035f4e90621eeba98861987b82318abcb25c): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[JohnTitor]

Small wins up to ~0.5%, there're 0.2% regressions but they can be noises or acceptable, I'm going to land.

@bors r=davidtwco

[bors]

📌 Commit 4f8e0eb has been approved by davidtwco

[bors]

⌛ Testing commit 4f8e0eb with merge 1a46283...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 1a46283 to master...