Auto merge of #115401 - Zoxc:freeze, r=oli-obk

Add `FreezeLock` type and use it to store `Definitions`

This adds a `FreezeLock` type which allows mutation using a lock until the value is frozen where it can be accessed lock-free. It's used to store `Definitions` in `Untracked` instead of a `RwLock`. Unlike the current scheme of leaking read guards this doesn't deadlock if definitions is written to after no mutation are expected.

Author: bors