Add test for soon to be deprecated 'class object' syntax

compiler/testData/diagnostics/tests/classObjects/deprecatedClassObjectSyntax.kt

@@ -0,0 +1,28 @@
+trait G
+
+class A {
+    class object A {
+
+    }
+}
+
+trait B {
+    class object : G {
+
+    }
+}
+
+fun main() {
+    A
+    A.A
+    g(B.Default)
+    g(B)
+
+    A.ext()
+    A.A.ext()
+}
+
+fun g(g: G) { <!UNUSED_EXPRESSION!>g<!> }
+
+fun A.A.ext() {
+}