[flang] Write function result to .mod file after dummy args

The function result can depend on the declaration of the dummy
arguments so it should be written to the .mod file after them.

For example:
```
  function f(x)
    integer :: x(:)
    integer :: f(size(x))
  end
```

Original-commit: flang-compiler/f18@f6c8c58c24368ae9f209b2c03637531ed869adeb
Reviewed-on: https://github.com/flang-compiler/f18/pull/650
Tree-same-pre-rewrite: false