Fix RegInit of Bundle lits (bp #1688) (#1689)

* Fix RegInit of Bundle lits (#1688)

Implemented by folding Element.ref into Data.ref. Element.ref had
special handling for literals, but because Bundles can also be literals,
there were code paths that tried to get the ref of a Bundle literal
which was non-existent. Now, all literals are handled together.

Because FIRRTL does not have support for Bundle literals, Bundle literal
refs are implemented by materializing a Wire.

(cherry picked from commit 5a6ce660

)

* Waive bincompat issue on package private method

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