[FIR] Add Stub kind for CFG

Edges of stub graphs shouldn't be completed

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraph.kt

@@ -35,6 +35,7 @@ class ControlFlowGraph(val declaration: FirDeclaration?, val name: String, val k
     internal fun complete() {
         assertState(State.Building)
         state = State.Completed
+        if (kind == Kind.Stub) return
         val sortedNodes = orderNodes()
         assert(sortedNodes.size == _nodes.size)
         for (node in _nodes) {
@@ -80,6 +81,7 @@ class ControlFlowGraph(val declaration: FirDeclaration?, val name: String, val k
         TopLevel(withBody = false),
         AnnotationCall(withBody = true),
         DefaultArgument(withBody = false),
+        Stub(withBody = true)
     }
 }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraphBuilder.kt

@@ -318,7 +318,7 @@ class ControlFlowGraphBuilder {
 
     fun enterClass() {
         pushGraph(
-            ControlFlowGraph(null, "STUB_CLASS_GRAPH", ControlFlowGraph.Kind.ClassInitializer),
+            ControlFlowGraph(null, "STUB_CLASS_GRAPH", ControlFlowGraph.Kind.Stub),
             mode = Mode.ClassInitializer
         )
     }