Port cfg_select! to the new attribute parsing system

[JonathanBrouwer]

Best reviewed commit by commit, since it involves some moving around of code

r? @jdonszelmann

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

[rustbot]

jdonszelmann is currently at their maximum review capacity.
They may take a while to respond.

[JonathanBrouwer]

@rustbot author
Need to address Folkert's comment

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[JonathanBrouwer]

@rustbot ready
Made the changes so this code is usable from rustfmt. I believe you should be able to call parse_cfg_select.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.