Expand `#[cfg]` and `#[cfg_attr]` left-to-right for consistency with other attributes

[Aaron1011]

Currently, cfg-stripping is performed before we invoke any proc-macro attributes. This means that #[my_attr] #[cfg(FALSE)] struct Foo {} does not invoke #[my_attr] at all, instead of invoking it with #[cfg(FALSE)] struct Foo {}

See https://github.com/rust-lang/rust/pull/82419/files/fc9d578bc5ada5162f49ed02a547cf87f2456a93#r580718089 for a more detailed example and discussion.

[petrochenkov]

I'm working on this right now.

[petrochenkov]

Crater run in #83354 showed that there's some non-negligible breakage.

In particular, some cases fundamentally require out-of-order expansion like any helper attributes inside cfg_attr

#[derive(Trait)]
#[cfg_attr(predicate, trait_helper)]
struct S;

#[test]
#[cfg_attr(predicate, ignore)]
fn my_test() {}

So they will require using #[cfg_eval] (#82679) if the left-to-right order is enforced by default.

[petrochenkov]

#92473 is the current iteration that attempts to implement some pre-requisites without changing observable language behavior.