Auto merge of #34216 - jseyfried:nested_cfg_attr, r=nrc
Support nested `cfg_attr` attributes
Support arbitrarily deeply nested `cfg_attr` attributes (e.g. `#[cfg_attr(foo, cfg_attr(bar, baz))]`).
This makes configuration idempotent.
Currently, the nighties do not support any `cfg_attr` nesting. Stable and beta support just one level of `cfg_attr` nesting (expect for attributes on macro-expanded nodes, where no nesting is supported).
This is a [breaking-change]. For example, the following would break:
```rust
macro_rules! m { () => {
#[cfg_attr(all(), cfg_attr(all(), cfg(foo)))]
fn f() {}
} }
m!();