[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_formatter/src/pattern/pattern_keyword.rs

@@ -9,6 +9,7 @@ impl FormatNodeRule<PatternKeyword> for FormatPatternKeyword {
     fn fmt_fields(&self, item: &PatternKeyword, f: &mut PyFormatter) -> FormatResult<()> {
         let PatternKeyword {
             range: _,
+            node_index: _,
             attr,
             pattern,
         } = item;

crates/ruff_python_formatter/src/pattern/pattern_match_as.rs

@@ -13,6 +13,7 @@ impl FormatNodeRule<PatternMatchAs> for FormatPatternMatchAs {
     fn fmt_fields(&self, item: &PatternMatchAs, f: &mut PyFormatter) -> FormatResult<()> {
         let PatternMatchAs {
             range: _,
+            node_index: _,
             pattern,
             name,
         } = item;

crates/ruff_python_formatter/src/pattern/pattern_match_class.rs

@@ -13,6 +13,7 @@ impl FormatNodeRule<PatternMatchClass> for FormatPatternMatchClass {
     fn fmt_fields(&self, item: &PatternMatchClass, f: &mut PyFormatter) -> FormatResult<()> {
         let PatternMatchClass {
             range: _,
+            node_index: _,
             cls,
             arguments,
         } = item;

crates/ruff_python_formatter/src/pattern/pattern_match_mapping.rs

@@ -21,6 +21,7 @@ impl FormatNodeRule<PatternMatchMapping> for FormatPatternMatchMapping {
             patterns,
             rest,
             range: _,
+            node_index: _,
         } = item;
 
         debug_assert_eq!(keys.len(), patterns.len());
@@ -163,6 +164,7 @@ fn find_double_star(pattern: &PatternMatchMapping, source: &str) -> Option<(Text
         patterns,
         rest,
         range: _,
+        node_index: _,
     } = pattern;
 
     // If there's no `rest` element, there's no `**`.

crates/ruff_python_formatter/src/pattern/pattern_match_or.rs

@@ -14,7 +14,11 @@ pub struct FormatPatternMatchOr;
 
 impl FormatNodeRule<PatternMatchOr> for FormatPatternMatchOr {
     fn fmt_fields(&self, item: &PatternMatchOr, f: &mut PyFormatter) -> FormatResult<()> {
-        let PatternMatchOr { range: _, patterns } = item;
+        let PatternMatchOr {
+            range: _,
+            node_index: _,
+            patterns,
+        } = item;
         let inner = format_with(|f: &mut PyFormatter| {
             let mut patterns = patterns.iter();
             let comments = f.context().comments().clone();

crates/ruff_python_formatter/src/pattern/pattern_match_sequence.rs

@@ -14,7 +14,11 @@ pub struct FormatPatternMatchSequence;
 
 impl FormatNodeRule<PatternMatchSequence> for FormatPatternMatchSequence {
     fn fmt_fields(&self, item: &PatternMatchSequence, f: &mut PyFormatter) -> FormatResult<()> {
-        let PatternMatchSequence { patterns, range } = item;
+        let PatternMatchSequence {
+            patterns,
+            range,
+            node_index: _,
+        } = item;
 
         let comments = f.context().comments().clone();
         let dangling = comments.dangling(item);

crates/ruff_python_formatter/src/pattern/pattern_match_value.rs

@@ -9,7 +9,11 @@ pub struct FormatPatternMatchValue;
 
 impl FormatNodeRule<PatternMatchValue> for FormatPatternMatchValue {
     fn fmt_fields(&self, item: &PatternMatchValue, f: &mut PyFormatter) -> FormatResult<()> {
-        let PatternMatchValue { value, range: _ } = item;
+        let PatternMatchValue {
+            value,
+            range: _,
+            node_index: _,
+        } = item;
         value.format().with_options(Parentheses::Never).fmt(f)
     }
 }