[FE] Add clear warning about future changes about nullability of safe call with non nullable receiver

^KT-46860

compiler/testData/diagnostics/tests/multimodule/duplicateMethod/simpleWithInheritance.kt

@@ -34,6 +34,6 @@ import p.*
 
 fun test(b: B?) {
     if (b is C && b is D) {
-        b<!UNNECESSARY_SAFE_CALL!>?.<!>getParent()
+        <!SAFE_CALL_WILL_CHANGE_NULLABILITY!>b<!UNNECESSARY_SAFE_CALL!>?.<!>getParent()<!>
     }
 }