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/emptyLhs.kt

@@ -0,0 +1,40 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
+
+val topLevelVal = 1
+fun topLevelFun() = 2
+
+val A.extensionVal: Int get() = 3
+fun A.extensionFun(): Int = 4
+
+class A {
+    val memberVal = 5
+    fun memberFun() = 6
+
+    val ok1 = ::topLevelVal
+    val ok2 = ::topLevelFun
+
+    fun fail1() {
+        ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>extensionVal<!>
+        ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>extensionFun<!>
+    }
+
+    fun fail2() {
+        ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>memberVal<!>
+        ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>memberFun<!>
+    }
+}
+
+
+
+val ok1 = ::topLevelVal
+val ok2 = ::topLevelFun
+
+fun A.fail1() {
+    ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>extensionVal<!>
+    ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>extensionFun<!>
+}
+
+fun A.fail2() {
+    ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>memberVal<!>
+    ::<!CALLABLE_REFERENCE_TO_MEMBER_OR_EXTENSION_WITH_EMPTY_LHS!>memberFun<!>
+}