gccrs: Introduce AST::Visitable class for AST

[dkm]

AST::Visitable is an abstract class with a unique accept_vis() method.
Make all abstract AST node class inherit from this class.

Allows for easy definition of operations on nodes that must accept a
visitor.

The static Dump::dump() is an example of such use: the static method
accepts any AST node, creates a Dump visitor and have it visit the AST
starting at the node.

This change also inserts a debug(Visitable&) function in the global
namespace to make it easy to call from the debugger (similar to
debug_tree or debug(rtx*) functions).

gcc/rust/ChangeLog:
            * ast/rust-ast-dump.cc (Dump::debug): New.
            * ast/rust-ast-dump.h (Dump::debug): Untemplate it.
            (debug): New.
            * ast/rust-ast.h (class Visitable): New.
            (class TokenTree): Inherit from Visitable.
            (class MacroMatch): Likewise.
            (class AttrInput): Likewise.
            (class MetaItemInner): Likewise.
            (class Pattern): Likewise.
            (classTypeParamBound): Likewise.
            (class GenericParam): Likewise.
            (class TraitItem): Likewise.
            (classInherentImplItem): Likewise.
            (class TraitImplItem): Likewise.
            (class ExternalItem): Likewise.
            (class SingleASTNode): Likewise.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>

[CohenArthur]

Great idea and great execution :) thank you!

[dkm]

Implemented your suggestion @CohenArthur