FIR: assume a lambda returns Unit if it ends with a non-expression

While it is theoretically useful to know that `{ while(true) {} }`
returns Nothing, CFG node deadness is not precise enough to do that: if
the entire lambda is dead, it's no longer possible to find out whether
the loop is terminating. Besides, `while (true)` and `if (true)` are
pretty much the only constructs like that anyway.

Note that this commit does not affect resolution for lambdas that end in
a Nothing-returning expression, e.g. `throw`.

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

@@ -13,6 +13,9 @@ import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.resolve.dfa.PersistentFlow
 import org.jetbrains.kotlin.fir.resolve.dfa.controlFlowGraph
+import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.fir.types.impl.FirImplicitNothingTypeRef
+import org.jetbrains.kotlin.fir.types.isNothing
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
 import org.jetbrains.kotlin.utils.SmartList
@@ -891,17 +894,18 @@ class AnnotationExitNode(owner: ControlFlowGraph, override val fir: FirAnnotatio
 
 // ----------------------------------- Stub -----------------------------------
 
-object FirStub : FirElement {
+object FirStub : FirExpression() {
     override val source: KtSourceElement? get() = null
+    override val typeRef: FirTypeRef = FirImplicitNothingTypeRef(null)
+    override val annotations: List<FirAnnotation> get() = listOf()
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {}
-
-    override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement {
-        return this
-    }
+    override fun <D> transformAnnotations(transformer: FirTransformer<D>, data: D): FirExpression = this
+    override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement = this
+    override fun replaceAnnotations(newAnnotations: List<FirAnnotation>) { assert(newAnnotations.isEmpty()) }
+    override fun replaceTypeRef(newTypeRef: FirTypeRef) { assert(newTypeRef.isNothing) }
 }
 
-
 // ----------------------------------- Smart-cast node -----------------------------------
 
 class SmartCastExpressionExitNode(owner: ControlFlowGraph, override val fir: FirSmartCastExpression, level: Int, id: Int) : CFGNode<FirSmartCastExpression>(owner, level, id) {