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.

----

This also adds a module, `syntax::ast_map::blocks`, that captures a
common abstraction shared amongst code blocks and procedure-like
things.  As part of this, moved `ast_map.rs` to subdir
`ast_map/mod.rs`, to follow our directory layout conventions.

(incorporated review feedback from huon, acrichto.)