Disallow references to extensions in classes

It's not clear in which order should the two receiver arguments be passed to
them, and there's no corresponding K*FunctionN class for now

 #KT-1183 In Progress

compiler/testData/diagnostics/tests/callableReference/extensionInClassDisallowed.kt

@@ -0,0 +1,14 @@
+class A {
+    fun Int.extInt() = 42
+    fun A.extA(x: String) = x
+    
+    fun main() {
+        ::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extInt<!>
+        ::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extA<!>
+    }
+}
+
+fun main() {
+    A::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extInt<!>
+    A::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extA<!>
+}