[flang] Enclose Walk overloads into class for lookup purposes (#115926)

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