If a tuple is passed as the only argument to a function that takes two unnamed arguments, we try to diagnose the missing argument label. However, in the old diagnose code we didn’t distinguish between non-existing arguments and arguments without labels; both behaved the same.
Thus, the missing second argument in the function call would line up with the empty second argument label, not producing a diagnostic, but hitting an assertion later on.
Distinguish between the two to fix the issue.
Fixes rdar://64319340
