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/property/classFromClass.kt

@@ -6,7 +6,7 @@ class A(var g: A) {
     val f: Int = 0
 
     fun test() {
-        val fRef: KProperty1<A, Int> = ::f
-        val gRef: KMutableProperty1<A, A> = ::g
+        val fRef: KProperty1<A, Int> = A::f
+        val gRef: KMutableProperty1<A, A> = A::g
     }
 }