Fix Reg() to properly handle clocks as rvalues (#3775) (#3780)

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

)

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