Are druids “shapechangers” for the purpose of a Polymorph spell saving throw?

The Polymorph spell states that “A shapechanger automatically succeeds on this saving throw”

Druids have Beast Shape, and although numerous textual references refer to this ability, at no point (that I can find) are they specifically called “shapechangers”. Now I know that I would rule “yes, they are” in a seat of my pants ruling, but is there a RAW 5e reference that makes this correlation and supports the case for Druids automatically saving from a Polymorph spell?