[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_test/src/assertion.rs

@@ -57,7 +57,7 @@ impl InlineFileAssertions {
     pub(crate) fn from_file(db: &Db, file: File) -> Self {
         let source = source_text(db, file);
         let lines = line_index(db, file);
-        let parsed = parsed_module(db, file);
+        let parsed = parsed_module(db, file).load(db);
         let comment_ranges = CommentRanges::from(parsed.tokens());
         Self {
             comment_ranges,

crates/ty_test/src/lib.rs

@@ -294,7 +294,7 @@ fn run_test(
     let failures: Failures = test_files
         .into_iter()
         .filter_map(|test_file| {
-            let parsed = parsed_module(db, test_file.file);
+            let parsed = parsed_module(db, test_file.file).load(db);
 
             let mut diagnostics: Vec<Diagnostic> = parsed
                 .errors()