Fix `Reg()` to properly handle clocks as rvalues (backport #3775) (#3778)

* Fix Reg() to properly handle clocks as rvalues (#3775)

Previously, it was not calling .ref. It was thus not properly:
1. Checking that the clock is bound hardware
2. Checking that the clock is visible in the current scope
3. Reifying the clock if its a view

Note that RegInit() was already correctly calling .ref.

(cherry picked from commit 1c348c55

)

# Conflicts:
#	src/test/scala/chiselTests/ClockSpec.scala

* Resolve backport conflicts

* fix test

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