Add a new check: checkReturnTypesHaveRefTypes [reviewed by lydia]

The two passes callDestructors and returnStarTuplesByRefArgs rely on the ref
type of a return type being defined.  This check verifies that it is present.

[tested with -compopts --verify in release/examples/primers/]


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22131 3a8e244f-b0f2-452b-bcba-4c88e055c3ca