[flang] Reorder the Walk functions.

Group the Walk functions that call WalkTupleClass, WalkUnionClass,
WalkWrapperClass all together, after the ones that don't follow a
simple pattern like that.

Fix the name of the include guard macro.

Original-commit: flang-compiler/f18@711613a4adaaf6083966b3993472dc74e97ba026
Reviewed-on: https://github.com/flang-compiler/f18/pull/15
Tree-same-pre-rewrite: false