[flang] Trait-based parse tree traversal.

Add a CLASS_TRAIT macro to idioms.h to define template metaprogramming
based on definable traits of user-defined classes.  Tweak the boilerplate
macros in parse-tree.h to add traits to the empty, wrapper, union, and
tuple classes in the parse tree.  Replace the macros in parse-tree-visitor.h
with trait-driven templates.

Original-commit: flang-compiler/f18@02400146fbebc8ef6d608699153146721a40dc37
Reviewed-on: https://github.com/flang-compiler/f18/pull/20
Tree-same-pre-rewrite: false