Rollup merge of #30863 - jseyfried:no_rc, r=eddyb

Use arena allocation instead of reference counting for `Module`s to fix memory leaks from `Rc` cycles.

A module references its module children and its import resolutions, and an import resolution references the module defining the imported name, so there is a cycle whenever a module imports something from an ancestor module.

For example,
```rust
mod foo { // `foo` references `bar`.
    fn baz() {}
    mod bar { // `bar` references the import.
        use foo::baz; // The import references `foo`.
    }
}
```

0 files changed, 0 insertions, 0 deletions