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.
niko actually mentioned to me back when I revised dataflow to work atop `cfg`: assuming we eventually revise dataflow to just use `cfgidx` indices directly, we should actually be able to get rid of this frozen/non-frozen distinction.  So I would just as soon not bother with the suffix since I hope to try to get rid of it soon anyway.