Allow underscore (unused) args in presence of kwargs (#58803)
Admittedly fixed because I thought I introduced this bug recently, but
actually, fix #32727. `f(_; kw) = 1` should now lower in a similar way
to `f(::Any; kw) = 1`, where we use a gensym for the first argument.
Not in this PR, but TODO: `nospecialize` underscore-only names
