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

^KT-46860

compiler/testData/diagnostics/testsWithStdLib/assert/cast.fir.kt

@@ -16,5 +16,5 @@ fun test1(a: A) {
 fun test2() {
     val a: A? = null;
     assert((a as B).bool())
-    a<!UNNECESSARY_SAFE_CALL!>?.<!>bool()
+    <!SAFE_CALL_WILL_CHANGE_NULLABILITY!>a<!UNNECESSARY_SAFE_CALL!>?.<!>bool()<!>
 }