Add trivial test for CStructVar declared in source code

kotlin-native/backend.native/tests/build.gradle

@@ -4034,6 +4034,10 @@ task interop_convert(type: KonanLocalTest) {
     source = "codegen/intrinsics/interop_convert.kt"
 }
 
+task interop_sourceCodeStruct(type: KonanLocalTest) {
+    source = "codegen/intrinsics/interop_sourceCodeStruct.kt"
+}
+
 /*
 TODO: This test isn't run automatically
 task interop_echo_server(type: RunInteropKonanTest) {

kotlin-native/backend.native/tests/codegen/intrinsics/interop_sourceCodeStruct.kt

@@ -0,0 +1,40 @@
+package codegen.intrinsics.interop_sourceCodeStruct
+
+import kotlinx.cinterop.*
+import kotlinx.cinterop.internal.*
+import kotlin.test.*
+
+// Just making sure this doesn't get accidentally forbidden or otherwise broken.
+// (however defining structs this way is still strongly discouraged, please define
+// structs in C headers or .def files instead).
+
+@CStruct("struct { int p0; int p1; }")
+class S(rawPtr: NativePtr) : CStructVar(rawPtr) {
+
+    companion object : CStructVar.Type(8, 4)
+
+    var x: Int
+        get() = memberAt<IntVar>(0).value
+        set(value) {
+            memberAt<IntVar>(0).value = value
+        }
+
+    var y: Int
+        get() = memberAt<IntVar>(4).value
+        set(value) {
+            memberAt<IntVar>(4).value = value
+        }
+}
+
+@Test
+fun test() = memScoped {
+    val s = alloc<S>()
+
+    s.x = 123
+    assertEquals(123, s.x)
+    assertEquals(123, s.ptr.reinterpret<IntVar>()[0])
+
+    s.y = 321
+    assertEquals(321, s.y)
+    assertEquals(321, s.ptr.reinterpret<IntVar>()[1])
+}