Add PartialDataView.supertype (backport #2826) (#2827)

* Add PartialDataView.supertype (#2826)

This factory method makes it easy to create PartialDataViews from a
Bundle type to its supertype. Because of the typing relationship, there
is no need to provide a mapping between fields. The only thing necessary
is to provide a function for constructing an instance of the supertype
from an instance of the subtype.

(cherry picked from commit 251d454a

)

# Conflicts:
#	core/src/main/scala/chisel3/experimental/dataview/package.scala
#	src/test/scala/chiselTests/experimental/DataView.scala

* Resolve backport conflicts

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