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 1c348c5)

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

core/src/main/scala/chisel3/Reg.scala

@@ -42,7 +42,7 @@ object Reg {
       requireIsChiselType(t, "reg type")
     }
     val reg = if (!t.mustClone(prevId)) t else t.cloneTypeFull
-    val clock = Node(Builder.forcedClock)
+    val clock = Builder.forcedClock.ref
 
     reg.bind(RegBinding(Builder.forcedUserModule, Builder.currentWhen))
     pushCommand(DefReg(sourceInfo, reg, clock))

src/test/scala/chiselTests/ClockSpec.scala

@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiselTests
+
+import chisel3._
+import chisel3.testers.BasicTester
+import circt.stage.ChiselStage
+
+class ClockAsUIntTester extends BasicTester {
+  assert(true.B.asClock.asUInt === 1.U)
+  assert(true.B.asClock.asBool === true.B)
+  stop()
+}
+
+class WithClockAndNoReset extends RawModule {
+  val clock1 = IO(Input(Clock()))
+  val clock2 = IO(Input(Clock()))
+  val in = IO(Input(Bool()))
+  val out = IO(Output(Bool()))
+  val a = withClock(clock2) {
+    RegNext(in)
+  }
+
+  out := a
+}
+
+class ClockSpec extends ChiselPropSpec {
+  property("Bool.asClock.asUInt should pass a signal through unaltered") {
+    assertTesterPasses { new ClockAsUIntTester }
+  }
+
+  property("Should be able to use withClock in a module with no reset") {
+    val circuit = ChiselStage.emitCHIRRTL(new WithClockAndNoReset)
+    circuit.contains("reg a : UInt<1>, clock2") should be(true)
+  }
+
+  property("Should be able to override the value of the implicit clock") {
+    val verilog = ChiselStage.emitSystemVerilog(new Module {
+      val gate = IO(Input(Bool()))
+      val in = IO(Input(UInt(8.W)))
+      val out = IO(Output(UInt(8.W)))
+      val gatedClock = (clock.asBool || gate).asClock
+      override protected def implicitClock = gatedClock
+
+      val r = Reg(UInt(8.W))
+      out := r
+      r := in
+    })
+    verilog should include("gatedClock = clock | gate;")
+    verilog should include("always @(posedge gatedClock)")
+  }
+
+  property("Should be able to add an implicit clock to a RawModule") {
+    val verilog = ChiselStage.emitSystemVerilog(new RawModule with ImplicitClock {
+      val foo = IO(Input(Bool()))
+      val in = IO(Input(UInt(8.W)))
+      val out = IO(Output(UInt(8.W)))
+      override protected val implicitClock = (!foo).asClock
+
+      val r = Reg(UInt(8.W))
+      out := r
+      r := in
+    })
+    verilog should include("always @(posedge implicitClock)")
+  }
+
+  property("Chisel should give a decent error message if you try to use a clock before defining it") {
+    val e = the[ChiselException] thrownBy (
+      ChiselStage.emitCHIRRTL(
+        new RawModule with ImplicitClock {
+          val r = Reg(UInt(8.W))
+          val foo = IO(Input(Clock()))
+          override protected def implicitClock = foo
+        },
+        args = Array("--throw-on-first-error")
+      )
+    )
+    e.getMessage should include(
+      "The implicit clock is null which means the code that sets its definition has not yet executed."
+    )
+  }
+
+  property("Chisel should give a decent error message if you use an unbound Clock") {
+    val e = the[ChiselException] thrownBy (
+      ChiselStage.emitCHIRRTL(
+        new RawModule {
+          withClock(Clock()) {
+            val r = Reg(UInt(8.W))
+          }
+        },
+        args = Array("--throw-on-first-error")
+      )
+    )
+    e.getMessage should include(
+      "'Clock' must be hardware, not a bare Chisel type. Perhaps you forgot to wrap it in Wire(_) or IO(_)?"
+    )
+  }
+
+  property("Chisel should give a decent error message if you use a Clock from another scope") {
+    val e = the[ChiselException] thrownBy (
+      ChiselStage.emitCHIRRTL(
+        new RawModule {
+          override def desiredName = "Parent"
+          val child = Module(new RawModule {
+            override def desiredName = "Child"
+            val clock = Wire(Clock())
+          })
+          withClock(child.clock) {
+            val r = Reg(UInt(8.W))
+          }
+        },
+        args = Array("--throw-on-first-error")
+      )
+    )
+    e.getMessage should include(
+      "operand 'Child.clock: Wire[Clock]' is not visible from the current module Parent"
+    )
+  }
+
+  property("Chisel should support Clocks from views") {
+    import chisel3.experimental.dataview._
+    val chirrtl = ChiselStage.emitCHIRRTL(new RawModule {
+      val clock = IO(Clock())
+      val view = clock.viewAs[Clock]
+      withClock(view) {
+        val r = Reg(UInt(8.W))
+      }
+    })
+    chirrtl should include("reg r : UInt<8>, clock")
+  }
+}