Extend --pretty flowgraph=ID to include dataflow results in output.

Use one or more of the following `-Z` flag options to tell the
graphviz renderer to include the corresponding dataflow sets (after
the iterative constraint propagation reaches a fixed-point solution):

  * `-Z flowgraph-print-loans` : loans computed via middle::borrowck
  * `-Z flowgraph-print-moves` : moves computed via middle::borrowck::move_data
  * `-Z flowgraph-print-assigns` : assignments, via middle::borrowck::move_data
  * `-Z flowgraph-print-all` : all of the available sets are included.

Fix #15016.
(hopefully the two methods will just be merged when the `frozen` distinction goes away.  but for now I added an extra line to point out that this, like the other `frozen` methods, is really only meant to be invoked post `propagate`.)