[ty] AST garbage collection (astral-sh#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 astral-sh/ty#214.

Author: ibraheemdev

Cargo.lock

@@ -51,6 +51,7 @@ aho-corasick = { version = "1.1.3" }
 anstream = { version = "0.6.18" }
 anstyle = { version = "1.0.10" }
 anyhow = { version = "1.0.80" }
+arc-swap = { version = "1.7.1" }
 assert_fs = { version = "1.1.0" }
 argfile = { version = "0.2.0" }
 bincode = { version = "2.0.0" }

Cargo.toml

@@ -21,6 +21,7 @@ ruff_source_file = { workspace = true }
 ruff_text_size = { workspace = true }
 
 anstyle = { workspace = true }
+arc-swap = { workspace = true }
 camino = { workspace = true }
 countme = { workspace = true }
 dashmap = { workspace = true }