[ty] AST garbage collection (#18482)

## Summary

Garbage collect ASTs once we are done checking a given file. Queries
with a cross-file dependency on the AST will reparse the file on demand.
This reduces ty's peak memory usage by ~20-30%.

The primary change of this PR is adding a `node_index` field to every
AST node, that is assigned by the parser. `ParsedModule` can use this to
create a flat index of AST nodes any time the file is parsed (or
reparsed). This allows `AstNodeRef` to simply index into the current
instance of the `ParsedModule`, instead of storing a pointer directly.

The indices are somewhat hackily (using an atomic integer) assigned by
the `parsed_module` query instead of by the parser directly. Assigning
the indices in source-order in the (recursive) parser turns out to be
difficult, and collecting the nodes during semantic indexing is
impossible as `SemanticIndex` does not hold onto a specific
`ParsedModuleRef`, which the pointers in the flat AST are tied to. This
means that we have to do an extra AST traversal to assign and collect
the nodes into a flat index, but the small performance impact (~3% on
cold runs) seems worth it for the memory savings.

Part of https://github.com/astral-sh/ty/issues/214.

crates/ruff_python_parser/src/lib.rs

@@ -208,13 +208,14 @@ pub fn parse_parenthesized_expression_range(
 ///
 /// ```
 /// use ruff_python_parser::parse_string_annotation;
-/// use ruff_python_ast::{StringLiteral, StringLiteralFlags};
+/// use ruff_python_ast::{StringLiteral, StringLiteralFlags, AtomicNodeIndex};
 /// use ruff_text_size::{TextRange, TextSize};
 ///
 /// let string = StringLiteral {
 ///     value: "'''\n int | str'''".to_string().into_boxed_str(),
 ///     flags: StringLiteralFlags::empty(),
 ///     range: TextRange::new(TextSize::new(0), TextSize::new(16)),
+///     node_index: AtomicNodeIndex::dummy()
 /// };
 /// let parsed = parse_string_annotation("'''\n int | str'''", &string);
 /// assert!(!parsed.is_ok());