This is just refactoring. SubprogramVisitor now contains the functionality
related to subprograms, including statement functions. This is preparation
for handling interface blocks.
Also, change MessageHandler to create the Messages object and return it
through an accessor rather than having it created outside and passed in.
This saves us from having to pass it up through the chain of constructors.
Original-commit: flang-compiler/f18@f73cfa2fe9
Reviewed-on: https://github.com/flang-compiler/f18/pull/88
Tree-same-pre-rewrite: false
3c1f2b9670