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/extensionFromClass.kt

@@ -1,5 +1,5 @@
 class A {
-    fun result() = (::foo)(this, "OK")
+    fun result() = (A::foo)(this, "OK")
 }
 
 fun A.foo(x: String) = x