Report error about val reassignment via backing field since Kotlin 1.3

Also improve message for current warning

 #KT-16681 Fixed

compiler/testData/diagnostics/tests/backingField/FieldReassignmentError.kt

@@ -0,0 +1,7 @@
+// !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
+
+val my: Int = 1
+    get() {
+        <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!>++
+        return field
+    }