[flang] Update C++style.md

Add guidelines for error messages and commas.  Add a version number for clang-format. Add spaces before ### sections to restart numbering.

Original-commit: flang-compiler/f18@3e8db49f34e2dc47ce5b89af6099710f708f64c3
Reviewed-on: https://github.com/flang-compiler/f18/pull/751
Tree-same-pre-rewrite: false