ast_map: Added iterator over all node id's that match a path suffix.

This is useful e.g. for tools need a node-id, such as the flowgraph
pretty printer, since it can avoids the need to first pretty-print the
whole expanded,identified input in order to find out what the node-id
actually is.

It currently only supports path suffixes thst are made up of module
names (e.g. you cannot use the type instantiation form `a::<int>::b`
or `option::Option::unwrap_or` as a path suffix for this tool, though
the tool will produce paths that have non-modulues in the portion of
the path that is not included in the suffix).

(addressed review feedback too)
refactored pprust::State constructor methods out from `pprust::print_crate`.

(Groundwork for pretty-printing only selected items in an input crate.)
Helper method for `pprust::State` for printing instances of `ast_map::Node`.
pretty-printer: let users choose particular items to pretty print.

With this change:

  * `--pretty variant=<node-id>` will print the item associated with
    `<node-id>` (where `<node-id>` is an integer for some node-id in
    the AST, and `variant` means one of {`normal`,`expanded`,...}).

  * `--pretty variant=<path-suffix>` will print all of the items that
    match the `<path-suffix>` (where `<path-suffix>` is a suffix of a
    path, and `variant` again means one of {`normal`,`expanded`,...}).

    Example 1: the suffix `typeck::check::check_struct` matches the
    item with the path `rustc::middle::typeck::check::check_struct`
    when compiling the `rustc` crate.

    Example 2: the suffix `and` matches `core::option::Option::and`
    and `core::result::Result::and` when compiling the `core` crate.

Both of the `--pretty variant=...` modes will include the full path to
the item in a comment that follows the item.

Note that when multiple paths match, then either:

  1. all matching items are printed, in series; this is what happens in
     the usual pretty-print variants, or

  2. the compiler signals an error; this is what happens in flowgraph
     printing.

----

Some drive-by improvements:

Heavily refactored the pretty-printing glue in driver.rs, introducing
a couple local traits to avoid cut-and-pasting very code segments that
differed only in how they accessed the `Session` or the
`ast_map::Map`. (Note the previous code had three similar calls to
`print_crate` which have all been unified in this revision; the
addition of printing individual node-ids exacerbated the situation
beyond tolerance.) We may want to consider promoting some of these
traits, e.g. `SessionCarrier`, for use more generally elsewhere in the
compiler; right now I have to double check how to access the `Session`
depending on what context I am hacking in.

Refactored `PpMode` to make the data directly reflect the fundamental
difference in the categories (in terms of printing source-code with
various annotations, versus printing a control-flow graph).

(also, addressed review feedback.)
pretty printer: Added some `run-make` tests of path-suffix lookup functionality.
auto merge of #16326 : pnkfelix/rust/fsk-add-path-suffix-lookup, r=huonw

Extended `ast_map::Map` with an iterator over all node id's that match a path suffix.

Extended pretty printer to let users choose particular items to pretty print, either by indicating an integer node-id, or by providing a path suffix.

 * Example 1: the suffix `typeck::check::check_struct` matches the item with the path `rustc::middle::typeck::check::check_struct` when compiling the `rustc` crate.

 * Example 2: the suffix `and` matches `core::option::Option::and` and `core::result::Result::and` when compiling the `core` crate.

Refactored `pprust` slightly to support the pretty printer changes.

(See individual commits for more description.)