Add OpaqueType support to Records (backport #2662) (#2679)

* Add OpaqueType support to Records (#2662)

OpaqueTypes are essentially type aliases that hide the underlying type.
They are implemented in Chisel as Records of a single, unnamed element
where the wrapping Record does not exist in the emitted FIRRTL.

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit df5afee2

)

* Fix test compilation

* Fix OpaqueType tests in RecordSpec

Need to implement cloneType correctly and to warn instead of error when
accessing .toTarget of non-hardware types because that is a warning (not
error) in 3.5.

* Waive MiMa false positives

Co-authored-by: Aditya Naik <91489422+adkian-sifive@users.noreply.github.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>