Auto merge of #33706 - jseyfried:refactor_cfg, r=nrc

Perform `cfg` attribute processing during macro expansion and fix bugs

This PR refactors `cfg` attribute processing and fixes bugs. More specifically:
 - It merges gated feature checking for stmt/expr attributes, `cfg_attr` processing, and `cfg` processing into a single fold.
  - This allows feature gated `cfg` variables to be used in `cfg_attr` on unconfigured items. All other feature gated attributes can already be used on unconfigured items.
 - It performs `cfg` attribute processing during macro expansion instead of after expansion so that macro-expanded items are configured the same as ordinary items. In particular, to match their non-expanded counterparts,
  - macro-expanded unconfigured macro invocations are no longer expanded,
  - macro-expanded unconfigured macro definitions are no longer usable, and
  - feature gated `cfg` variables on macro-expanded macro definitions/invocations are now errors.

This is a [breaking-change]. For example, the following would break:
```rust
macro_rules! m {
    () => {
        #[cfg(attr)]
        macro_rules! foo { () => {} }
        foo!(); // This will be an error

        macro_rules! bar { () => { fn f() {} } }
        #[cfg(attr)] bar!(); // This will no longer be expanded ...
        fn g() { f(); } // ... so that `f` will be unresolved.

        #[cfg(target_thread_local)] // This will be a gated feature error
        macro_rules! baz { () => {} }
    }
}

m!();
```

r? @nrc

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/codemap.rs
Simple merge

diff --cc src/libsyntax/ext/expand.rs
Simple merge