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.
`into_string` would be more efficient (even more efficient would be preallocating with `String::with_capacity` and `push_str`'ing to built it... but that's a lot more effort than adding two characters, likely for little gain).