Make it legal for concrete resets to drive abstract reset (backport #2018) (#2029)

* Make it legal for concrete resets to drive abstract reset (#2018)

This has been legal in FIRRTL since v1.2.3 (when reset inference started
using a unification-style algorithm) but was never exposed in the Chisel
API.

Also delete the overridden connects in AsyncReset and ResetType which
just duplicate logic from MonoConnect.

(cherry picked from commit 4b7b771e

)

* Fix issues with tests

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