[flang] Fix classes with variant and other fields

Fortran::FormatItem and UseStmt have both normal fields and a variant
named 'u'. The Walk calls for the normal fields were missing.
These were the only two classes I found where this occurs.

Original-commit: flang-compiler/f18@23b8695a74631e0207f7c87be5c602896a9f0880
Reviewed-on: https://github.com/flang-compiler/f18/pull/17