Make anonymous classes for callable references synthetic

#KT-28453 Fixed

compiler/testData/codegen/box/callableReference/classesAreSynthetic.kt

@@ -0,0 +1,20 @@
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND: JVM_IR
+// WITH_RUNTIME
+
+class C {
+    fun f() {}
+    val r = 0
+}
+
+fun check(c: Class<*>) {
+    if (!c.isSynthetic) throw AssertionError("Fail: $c is not synthetic: ${c.modifiers}")
+}
+
+fun box(): String {
+    check(C::f.javaClass)
+    check(C::r.javaClass)
+    check(C()::f.javaClass)
+    check(C()::r.javaClass)
+    return "OK"
+}