[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/ruff_python_formatter/src/lib.rs

@@ -165,7 +165,7 @@ where
 pub fn formatted_file(db: &dyn Db, file: File) -> Result<Option<String>, FormatModuleError> {
     let options = db.format_options(file);
 
-    let parsed = parsed_module(db.upcast(), file);
+    let parsed = parsed_module(db.upcast(), file).load(db.upcast());
 
     if let Some(first) = parsed.errors().first() {
         return Err(FormatModuleError::ParseError(first.clone()));
@@ -174,7 +174,7 @@ pub fn formatted_file(db: &dyn Db, file: File) -> Result<Option<String>, FormatM
     let comment_ranges = CommentRanges::from(parsed.tokens());
     let source = source_text(db.upcast(), file);
 
-    let formatted = format_node(parsed, &comment_ranges, &source, options)?;
+    let formatted = format_node(&parsed, &comment_ranges, &source, options)?;
     let printed = formatted.print()?;
 
     if printed.as_code() == &*source {