FIR checker: report UNSAFE_CALL for overloaded function calls

Previously if an unsafe call is to an overloaded function, FIR checkers
report NONE_APPLICABLE. This change instead report them as UNSAFE_CALL
or its variants.

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/43.fir.kt

@@ -14,7 +14,7 @@ fun case_1(x: Double?, y: Double?) : Double {
     } else if (x == null && y != null) {
         y
     } else {
-        x <!NONE_APPLICABLE!>+<!> y
+        x <!UNSAFE_OPERATOR_CALL!>+<!> y
     }
 }