[flang] Change formatting of common::visitors{}

Including a comma after the last lambda in a `common::visitors{}` list
causes clang-format to do a better job of formatting them.

Add that recommendation to C++style.md, insert the missing commas,
and reformat the changed files.

Original-commit: flang-compiler/f18@a2486ca3b6daf6cfc3a5cf95ad89719297d04b72
Reviewed-on: https://github.com/flang-compiler/f18/pull/232