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

^KT-46860

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

@@ -1,5 +1,5 @@
 // FIR_IDENTICAL
-// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL -SAFE_CALL_WILL_CHANGE_NULLABILITY
 
 // MODULE: m0
 // FILE: a.kt
@@ -41,4 +41,4 @@ fun test(b: B?) {
     if (b is C) {
         b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>foo<!>(null, null)
     }
-}
+}