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/privateClassMember.kt

@@ -1,7 +1,7 @@
 class A {
     private fun foo() = "OK"
 
-    fun bar() = (::foo)(this)
+    fun bar() = (A::foo)(this)
 }
 
 fun box() = A().bar()