Enable Clock Invalidation (#2485)

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 5d8a0c8)

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

core/src/main/scala/chisel3/Clock.scala

@@ -22,7 +22,7 @@ sealed class Clock(private[chisel3] val width: Width = Width(1)) extends Element
     this.getClass == that.getClass
 
   override def connect(that: Data)(implicit sourceInfo: SourceInfo, connectCompileOptions: CompileOptions): Unit = that match {
-    case _: Clock => super.connect(that)(sourceInfo, connectCompileOptions)
+    case _: Clock | DontCare => super.connect(that)(sourceInfo, connectCompileOptions)
     case _ => super.badConnect(that)(sourceInfo)
   }
 

src/test/scala/chiselTests/InvalidateAPISpec.scala

@@ -214,4 +214,12 @@ class InvalidateAPISpec extends ChiselPropSpec with Matchers with BackendCompila
     val firrtlOutput = myGenerateFirrtl(new ModuleWithoutDontCare)
     firrtlOutput should include("is invalid")
   }
+
+  property("a clock should be able to be connected to a DontCare") {
+    class ClockConnectedToDontCare extends Module {
+      val io = IO(new Bundle { val foo = Output(Clock()) })
+      io.foo := DontCare
+    }
+    myGenerateFirrtl(new ClockConnectedToDontCare) should include("io.foo is invalid")
+  }
 }