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.
The name is indeed weird.  I guess I was biased by the amount of implementation code in there dedicated to `FnLike`, but another name would actually be nicer, if for no other reason than to avoid the stuttering `fn_like::FnLike.`

So a mini-bikeshed:
* `syntax::ast_map::flow_unit`, as you suggested
* `syntax::ast_map::blocks`,
* `syntax::ast_map::code_bodies`,
* `syntax::ast_map::fn_like_bodies`

(I really liked `code_units` at one point, but I think unicode gets to reserve that pair of words.)