naming an fn parameter __unused breaks codegen #3187

Admittedly a very, very tiny edge case but still, it feels morally wrong:

abuseUnused :: forall a. a -> a
abuseUnused __unused = __unused

var abuseUnused = function () {
    return __unused;
};

The culprit here is the removeUnusedArg optimization, which bases its functionality off the name of the argument.

Potential easy solution: change the constant to an illegal name?