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"
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java

@@ -2782,6 +2782,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
                 doTestWithStdlib(fileName);
             }
 
+            @TestMetadata("syntheticFields.kt")
+            public void testSyntheticFields() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/mapping/syntheticFields.kt");
+                doTestWithStdlib(fileName);
+            }
+
             @TestMetadata("topLevelProperty.kt")
             public void testTopLevelProperty() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/mapping/topLevelProperty.kt");

core/reflection.jvm/src/kotlin/reflect/jvm/mapping.kt

@@ -118,12 +118,15 @@ public val Class<*>.kotlinPackage: KPackage
     get() = KPackageImpl(this)
 
 
-// TODO: make nullable to filter out synthetic fields (KT-5759)
 /**
- * Returns a [KProperty] instance corresponding to the given Java [Field] instance.
+ * Returns a [KProperty] instance corresponding to the given Java [Field] instance,
+ * or `null` if this field cannot be represented by a Kotlin property
+ * (for example, if it is a synthetic field).
  */
-public val Field.kotlin: KProperty<*>
+public val Field.kotlin: KProperty<*>?
     get() {
+        if (isSynthetic()) return null
+
         val clazz = getDeclaringClass()
         val name = getName()
         val modifiers = getModifiers()