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/classMemberFromExtension.kt

@@ -3,7 +3,7 @@ class A {
     fun k(k: Int) = k
 }
 
-fun A.foo() = (::o)(this) + (A::k)(this, 222)
+fun A.foo() = (A::o)(this) + (A::k)(this, 222)
 
 fun box(): String {
     val result = A().foo()