Add an AST node type enum to AST nodes.

When inspecting an AST node in the debugger, it can be difficult to
find its concrete type.  Add a "node_type" virtual function to all the
concrete AST nodes.

Use a macro to generate the definition of node_type as well as Accept
and the AsXXX type testing/casting functions.

Review URL: http://codereview.chromium.org/3431026

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5549 ce2b1a6d-e550-0410-aec6-3dcde31c8c00