Make `toTarget` fail if called on a Literal (or would otherwise not serialize...

Make `toTarget` fail if called on a Literal (or would otherwise not serialize properly) (#1714) (#1721)

* Add (failing) Test for Data toTarget calls

Add scaladoc and clean up test

* Builder: don't let .toTarget pass if it won't be able to deserialize properly later

* Update src/test/scala/chiselTests/ReferenceTargetSpec.scala

* Rename and simplify tests for literal toTarget

(cherry picked from commit b578aa5e

)

Co-authored-by: Megan Wachs <megan@sifive.com>