Add support for Vec literals of empty Vecs (backport #4070) (#4072)

* Add support for Vec literals of empty Vecs (#4070)

Also tweak Vec.Lit code to use cloneAsSuperType rather than implementing
its own version of the same concept.

(cherry picked from commit 86082b1a

)

# Conflicts:
#	src/test/scala/chiselTests/BundleLiteralSpec.scala
#	src/test/scala/chiselTests/VecLiteralSpec.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>