Apply interop lowering to new declarations it produces

#KT-36182 Fixed

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/InteropLowering.kt

@@ -815,7 +815,17 @@ private class InteropLoweringPart2(val context: Context) : FileLoweringPass {
         val transformer = InteropTransformer(context, irFile)
         irFile.transformChildrenVoid(transformer)
 
-        irFile.addChildren(transformer.newTopLevelDeclarations)
+        while (transformer.newTopLevelDeclarations.isNotEmpty()) {
+            val newTopLevelDeclarations = transformer.newTopLevelDeclarations.toList()
+            transformer.newTopLevelDeclarations.clear()
+
+            // Assuming these declarations contain only new IR (i.e. existing lowered IR has not been moved there).
+            // TODO: make this more reliable.
+            val loweredNewTopLevelDeclarations =
+                    newTopLevelDeclarations.map { it.transform(transformer, null) as IrDeclaration }
+
+            irFile.addChildren(loweredNewTopLevelDeclarations)
+        }
     }
 }
 

backend.native/tests/interop/basics/callbacksAndVarargs.kt

@@ -7,13 +7,22 @@ import kotlin.test.*
 import kotlinx.cinterop.*
 import ccallbacksAndVarargs.*
 
-fun main(args: Array<String>) {
+fun main() {
+    testStructCallbacks()
+    testVarargs()
+    testCallableReferences()
+}
+
+fun testStructCallbacks() {
     assertEquals(42, getX(staticCFunction { -> cValue<S> { x = 42 } }))
     applyCallback(cValue { x = 17 }, staticCFunction { it: CValue<S> ->
         assertEquals(17, it.useContents { x })
     })
 
     assertEquals(66, makeS(66, 111).useContents { x })
+}
+
+fun testVarargs() {
     assertEquals(E.ONE, makeE(1))
 
     getVarargs(
@@ -51,3 +60,11 @@ fun main(args: Array<String>) {
         assertEquals(null, a15)
     }
 }
+
+fun testCallableReferences() {
+    val sumRef = ::sum
+    assertEquals(3, sumRef(1, 2))
+
+    val sumPtr = staticCFunction(::sum)
+    assertEquals(7, sumPtr(3, 4))
+}

backend.native/tests/interop/basics/ccallbacksAndVarargs.def

@@ -72,3 +72,7 @@ static struct Args getVarargs(int ignore, ...) {
 
     return result;
 }
+
+static int sum(int first, int second) {
+    return first + second;
+}