[flang] Preserve some minor changes that accumulated as I was implementing

[flang] Preserve some minor changes that accumulated as I was implementing
an unparsing feature.

Enum classes in parse tree node class types are now defined with
a macro so that the names of the enumerations can be captured for
formatting purposes.

Add a TODO comment.

Rebase to new master with parse tree traversal code.

Original-commit: flang-compiler/f18@d6bfcfd526cc8464046e855181b7771631793313
Reviewed-on: https://github.com/flang-compiler/f18/pull/16