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) (bp #1714) (#1720)

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

* 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

)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

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

Co-authored-by: Megan Wachs <megan@sifive.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>