[flang] Merge parse-tree-mutator.h into parse-tree-visitor.h

Use the latter for visiting and mutating and delete the former.

Original-commit: flang-compiler/f18@ac5a098ca7187eeb5d5bd2104c8d29363075b368
Reviewed-on: https://github.com/flang-compiler/f18/pull/60
Tree-same-pre-rewrite: false