[syntax-errors] Async comprehension in sync comprehension (#17177)

Summary
--

Detect async comprehensions nested in sync comprehensions in async
functions before Python 3.11, when this was [changed].

The actual logic of this rule is very straightforward, but properly
tracking the async scopes took a bit of work. An alternative to the
current approach is to offload the `in_async_context` check into the
`SemanticSyntaxContext` trait, but that actually required much more
extensive changes to the `TestContext` and also to ruff's semantic
model, as you can see in the changes up to
31554b473507034735bd410760fde6341d54a050. This version has the benefit
of mostly centralizing the state tracking in `SemanticSyntaxChecker`,
although there was some subtlety around deferred function body traversal
that made the changes to `Checker` more intrusive too (hence the new
linter test).

The `Checkpoint` struct/system is obviously overkill for now since it's
only tracking a single `bool`, but I thought it might be more useful
later.

[changed]: https://github.com/python/cpython/issues/77527

Test Plan
--

New inline tests and a new linter integration test.

crates/ruff_linter/resources/test/fixtures/syntax_errors/async_comprehension.ipynb

@@ -0,0 +1,57 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "6a70904e-dbfe-441c-99ec-12e6cf57f8ba",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "async def elements(n): yield n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "5412fc2f-76eb-42c0-8db1-b5af6fdc46aa",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "[x async for x in elements(5)]  # okay, async at top level"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "dc3c94a7-2e64-42de-9351-260b3f41c3fd",
+   "metadata": {
+    "scrolled": true
+   },
+   "outputs": [],
+   "source": [
+    "[[x async for x in elements(5)] for i in range(5)]  # error on 3.10, okay after"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.10.16"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}