Error messages about "var with open / custom setter initialization" changed accordingly with the new backing field syntax

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

@@ -0,0 +1,11 @@
+class Flower() {
+
+    var minusOne: Int = 1
+        get() = field + 1
+        set(n: Int) { field = n - 1 }
+
+    var oldSyntax: Int = 1
+        get() = <!BACKING_FIELD_OLD_SYNTAX!>$oldSyntax<!> - 1
+        set(arg) { <!BACKING_FIELD_OLD_SYNTAX!>$oldSyntax<!> = arg + 1 }
+
+}