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

idea/testData/unifier/equivalence/expressions/callableReferences/classRefRuntime.kt

@@ -4,8 +4,8 @@ class A {
     inner class C
 
     fun foo() {
-        <selection>::B</selection>
-        ::C
+        <selection>A::B</selection>
+        A::C
         A::B
         A::C
     }