Add Freeze type and use it to store Definitions

compiler/rustc_data_structures/src/sync.rs

@@ -61,6 +61,9 @@ pub use vec::{AppendOnlyIndexVec, AppendOnlyVec};
 
 mod vec;
 
+mod freeze;
+pub use freeze::{Freeze, FreezeReadGuard, FreezeWriteGuard};
+
 mod mode {
     use super::Ordering;
     use std::sync::atomic::AtomicU8;