reuse available belongs_to? method

Reflection has a `belongs_to?` method. Instead of checking for
`macro == :belongs_to` throughout the source reuse existing
method.

I also bumped `foreign_key_present?` method onto on line because
the `belongs_to?` makes it shorter than other longer lines in
the same class.
Merge pull request #15499 from eileencodes/reuse-available-belongs_to-method

reuse available belongs_to? method