[flang] Enclose Walk overloads into class for lookup purposes

The parse-tree-visitor consists of a range of Walk functions where each
overload is specialized for a particular case. These overloads do call
one another, and due to the usual name lookup rules, an earlier overload
can't call an overload defined later unless the latter was declared
ahead of time.
To avoid listing a number of declarations at the beginning of the header
enclose them in a class as static members, with a couple of simple
forwarding calls. This takes advantage of the class member name lookup,
which uses the entire class definition for lookup.

Author: kparzysz