[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_python_parser/resources/inline/ok/all_async_comprehension_py310.py

@@ -0,0 +1,2 @@
+# parse_options: {"target-version": "3.10"}
+async def test(): return [[x async for x in elements(n)] async for n in range(3)]

crates/ruff_python_parser/resources/inline/ok/nested_async_comprehension_py310.py

@@ -0,0 +1,9 @@
+# parse_options: {"target-version": "3.10"}
+# this case fails if exit_expr doesn't run
+async def f():
+    [_ for n in range(3)]
+    [_ async for n in range(3)]
+# and this fails without exit_stmt
+async def f():
+    def g(): ...
+    [_ async for n in range(3)]

crates/ruff_python_parser/resources/inline/ok/nested_async_comprehension_py311.py

@@ -0,0 +1,4 @@
+# parse_options: {"target-version": "3.11"}
+async def f(): return [[x async for x in foo(n)] for n in range(3)]    # list
+async def g(): return [{x: 1 async for x in foo(n)} for n in range(3)] # dict
+async def h(): return [{x async for x in foo(n)} for n in range(3)]    # set