[FIR] Add INAPPLICABLE_OPERATOR_MODIFIER diagnostic

compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.fir.kt

@@ -1,7 +1,7 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 class C {
-    operator fun compareTo(c: C): Int? = null
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(c: C): Int? = null
 }
 
 fun test(c: C) {
@@ -9,4 +9,4 @@ fun test(c: C) {
     c <= c
     c >= c
     c > c
-}
+}