Don't create KProperty instance for synthetic field

#KT-5759 Fixed

compiler/testData/codegen/boxWithStdlib/reflection/mapping/syntheticFields.kt

@@ -0,0 +1,14 @@
+import kotlin.reflect.jvm.kotlin
+
+class A {
+    // There's a synthetic "$kotlinClass" field here
+}
+
+fun box(): String {
+    for (field in javaClass<A>().getDeclaredFields()) {
+        val prop = field.kotlin
+        if (prop != null) return "Fail, property found: $prop"
+    }
+
+    return "OK"
+}