[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/semantic_model.rs

@@ -232,7 +232,7 @@ mod tests {
 
         let foo = system_path_to_file(&db, "/src/foo.py").unwrap();
 
-        let ast = parsed_module(&db, foo);
+        let ast = parsed_module(&db, foo).load(&db);
 
         let function = ast.suite()[0].as_function_def_stmt().unwrap();
         let model = SemanticModel::new(&db, foo);
@@ -251,7 +251,7 @@ mod tests {
 
         let foo = system_path_to_file(&db, "/src/foo.py").unwrap();
 
-        let ast = parsed_module(&db, foo);
+        let ast = parsed_module(&db, foo).load(&db);
 
         let class = ast.suite()[0].as_class_def_stmt().unwrap();
         let model = SemanticModel::new(&db, foo);
@@ -271,7 +271,7 @@ mod tests {
 
         let bar = system_path_to_file(&db, "/src/bar.py").unwrap();
 
-        let ast = parsed_module(&db, bar);
+        let ast = parsed_module(&db, bar).load(&db);
 
         let import = ast.suite()[0].as_import_from_stmt().unwrap();
         let alias = &import.names[0];