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/kt7564.kt

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