Use FnvHashMap instead of HashMap in rustc
auto merge of #18821 : arielb1/rust/fnv-hash-map, r=eddyb

This should improve performance
rustc: do not inherit #[stable]

This patch tweaks the stability inheritance infrastructure so that
`#{stable]` attributes are not inherited. Doing so solves two problems:

1. It allows us to mark module *names* as stable without accidentally
marking the items they contain as stable.

2. It means that a `#[stable]` attribution must always appear directly
on the item it applies to, which makes it easier for reviewers to catch
changes to stable APIs.

Fixes #17484
Update tests for new stability inheritance rules