This crash happens because shouldAddSelfFixit (a check for the expected_self_before_reference diagnotic) performs name lookup inside the parser, which causes a search for extensions, which triggers the mangler, which can get confused if we are in an unfinished declaration (a PatternBindingInitilizer in this case).
A cleaner fix would be to move that logic inside Sema, but it would require much more refactoring that I don’t feel comfortable doing. Instead, this band-aid checks if the innermost type is inside a local context. If that’s the case, we can ignore the search for extensions.
