[flang] Add ENUM_CLASS to define enum class with ToString function.

This replaces DEFINE_NESTED_ENUM_CLASS in parse-tree.h but works
similarly. "ENUM_CLASS(Foo, A, B, C)" defined enum class Foo with
enumerators A, B, C. It also defines an overloading of EnumToString
that converts enumerators to their string representation.

Change unparse.cc to adapt to this change.

Make use of ENUM_CLASS in attr.h and attr.cc.

Original-commit: flang-compiler/f18@c45b8f172a2559e22d1aaf12257b4e2558df8c6f
Reviewed-on: https://github.com/flang-compiler/f18/pull/31