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/noAmbiguityMemberVsTopLevel.kt

@@ -8,7 +8,7 @@ class A {
     fun foo() {}
     
     fun main() {
-        val x = ::foo
+        val x = ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>foo<!>
 
         checkSubtype<KFunction1<A, Unit>>(x)
     }