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/codegen/boxWithStdlib/callableReference/function/innerConstructorFromExtension.kt

@@ -5,7 +5,7 @@ class A {
     }
 }
 
-fun A.foo() = (A::Inner)(this).o + (::Inner)(this).k
+fun A.foo() = (A::Inner)(this).o + (A::Inner)(this).k
 
 fun box(): String {
     val result = A().foo()