[syntax-errors] `await` outside async functions (#17363)
Summary
--
This PR implements detecting the use of `await` expressions outside of
async functions. This is a reimplementation of
[await-outside-async
(PLE1142)](https://docs.astral.sh/ruff/rules/await-outside-async/) as a
semantic syntax error.
Despite the rule name, PLE1142 also applies to `async for` and `async
with`, so these are covered here too.
Test Plan
--
Existing PLE1142 tests.
I also deleted more code from the `SemanticSyntaxCheckerVisitor` to
avoid changes in other parser tests.
