[flang] Simplify name resolution visitor for parser::Enumerator

Do not define a Pre and a Post, simply walk the expression
with Walk in Pre to resolve the names inside the expression
before evaluating it.

Original-commit: flang-compiler/f18@b0e3c1691e58f968f01e9aa6d622e6f31c04f13b
Reviewed-on: https://github.com/flang-compiler/f18/pull/689
Tree-same-pre-rewrite: false