[flang] Add MessageHandler::Say2.

This handles the common case of an error message referring to
a name, and an attached message referring to another name.
For example, reporting an error where a name is already declared
and mentioning the previous declaration.

Original-commit: flang-compiler/f18@901b50dcf2860c5fcd5f94f0198981fe871d6716
Reviewed-on: https://github.com/flang-compiler/f18/pull/97
Tree-same-pre-rewrite: false