Forbid callable references to members and extensions with empty LHS

This syntax is reserved to be likely used in the future as a shorthand for
"this::foo" where the resulting expression doesn't take the receiver as a
parameter but has "this" already bound to it

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

@@ -4,8 +4,8 @@ class A {
     fun A.extA(x: String) = x
     
     fun main() {
-        ::<!MISSING_RECEIVER, EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extInt<!>
-        ::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extA<!>
+        Int::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extInt<!>
+        A::<!EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED!>extA<!>
     }
 }