Rollup merge of #139035 - nnethercote:PatKind-Missing, r=oli-obk

Add new `PatKind::Missing` variants

To avoid some ugly uses of `kw::Empty` when handling "missing" patterns, e.g. in bare fn tys. Helps with #137978. Details in the individual commits.

r? ``@oli-obk``

Author: Zalathar