Save/restore when+layer stack w/D/I, fix crashes. (#4442)

Add tests that demonstrate crashing without this change.

Fixes #4441.

core/src/main/scala/chisel3/ModuleImpl.scala

@@ -166,8 +166,12 @@ private[chisel3] trait ObjectModuleImpl {
     implicit sourceInfo: SourceInfo
   ): T = {
     val parent = Builder.currentModule
+    val whenStackOpt = Option.when(Builder.hasDynamicContext)(Builder.whenStack)
+    val layerStackOpt = Option.when(Builder.hasDynamicContext)(Builder.layerStack)
     val module: T = bc // bc is actually evaluated here
     if (!parent.isEmpty) { Builder.currentModule = parent }
+    whenStackOpt.foreach(Builder.whenStack = _)
+    layerStackOpt.foreach(Builder.layerStack = _)
 
     module
   }

src/test/scala/chiselTests/experimental/hierarchy/InstanceSpec.scala

@@ -78,6 +78,30 @@ class InstanceSpec extends ChiselFunSpec with Utils {
       val chirrtl = circt.stage.ChiselStage.emitCHIRRTL(new Top)
       chirrtl should include("connect i0.realIn.aminusx, UInt<7>(0h7b)")
     }
+    it("(0.f): access under when does not crash") {
+      class Top extends Module {
+        val definition = Definition(new AddTwo)
+        val i0 = Instance(definition)
+        when(true.B) {
+          val i = i0.i0 // This should not error
+        }
+      }
+      circt.stage.ChiselStage.emitCHIRRTL(new Top)
+    }
+    it("(0.g): access under layer does not crash") {
+      object A extends layer.Layer(layer.LayerConfig.Extract())
+      class Top extends Module {
+        val definition = Definition(new AddTwo)
+        val i0 = Instance(definition)
+        layer.block(A) {
+          val i = i0.i0 // This should not error
+        }
+        layer.block(A) {
+          val i = i0.i1 // This should not error
+        }
+      }
+      circt.stage.ChiselStage.emitCHIRRTL(new Top)
+    }
   }
   describe("(1) Annotations on instances in same chisel compilation") {
     it("(1.a): should work on a single instance, annotating the instance") {