detect when switch on error shadows it's own switch case capture

[Techatrix]

zig ast-check fails to report the right error message for this code:

e catch |err| switch (err) {
    error.Foo => |err| {
        _ = err catch {};
    },
};

It does not detect that there are two declarations of err and reports that the inner err capture isn't used. This bug happens with the "switch on captured error" syntax pattern which checked the wrong scope for shadowing.

[Techatrix]

Anyone got an idea what would cause the x86_64-windows-release CI to just hang?

[andrewrk]

We've really suffered too long without #19821 being implemented

[mlugg]

Triggered a re-run since that timeout... probably wasn't your fault? We'll see!