[SR-4978] Fix compiler crash for malformed ASTs

[ghost]

Resolves SR-4978.

[ghost]

@swift-ci Please smoke test

[slavapestov]

Do you mind adding a test case?

[ghost]

New to this but I will try to have a go at it

[jrose-apple]

Thanks, Nate! I think for the test you'd just want to have a file with a bunch of those declarations at the top level (init, deinit, case etc.). You can add them to tests/NameBinding/reference-dependencies-errors.swift, then check that the test fails before your change and succeeds after it. (There's information on how to run a single test in docs/Testing.rst.)

[ghost]

Thank you Jordan for the direction. Tested locally.

[CodaFi]

@swift-ci please smoke test