[ty] Add infrastructure for AST garbage collection (#18445)

## Summary

https://github.com/astral-sh/ty/issues/214 will require a couple
invasive changes that I would like to get merged even before garbage
collection is fully implemented (to avoid rebasing):
- `ParsedModule` can no longer be dereferenced directly. Instead you
need to load a `ParsedModuleRef` to access the AST, which requires a
reference to the salsa database (as it may require re-parsing the AST if
it was collected).
- `AstNodeRef` can only be dereferenced with the `node` method, which
takes a reference to the `ParsedModuleRef`. This allows us to encode the
fact that ASTs do not live as long as the database and may be collected
as soon a given instance of a `ParsedModuleRef` is dropped. There are a
number of places where we currently merge the `'db` and `'ast`
lifetimes, so this requires giving some types/functions two separate
lifetime parameters.

crates/ty_python_semantic/src/suppression.rs

@@ -88,7 +88,7 @@ declare_lint! {
 
 #[salsa::tracked(returns(ref))]
 pub(crate) fn suppressions(db: &dyn Db, file: File) -> Suppressions {
-    let parsed = parsed_module(db.upcast(), file);
+    let parsed = parsed_module(db.upcast(), file).load(db.upcast());
     let source = source_text(db.upcast(), file);
 
     let mut builder = SuppressionsBuilder::new(&source, db.lint_registry());