[flang] Add common base class for context, Walk(), and Say()

We need access to the SemanticsContext and Walk() function everywhere,
so move them to a BaseVisitor, a base class at the root of the class
hierarchy.

Also move MessageHandler to be a data member of BaseVisitor and forward
Say calls to it.

Original-commit: flang-compiler/f18@eb9adc342a5495b0070a14d163f55209c63623d7
Reviewed-on: https://github.com/flang-compiler/f18/pull/238
Tree-same-pre-rewrite: false