Enable Clock Invalidation (#2485) (#2486)

Loosen restrictions on clocks to enable them to be connected to
DontCare, i.e., be invalidated.

Co-authored-by: Jack Koenig <koenig@sifive.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit 5d8a0c8e

)

# Conflicts:
#	core/src/main/scala/chisel3/Clock.scala

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>