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/testData/diagnostics/tests/smartCasts/varnotnull/plusplusMinusminus.fir.kt

@@ -15,7 +15,7 @@ fun bar(arg: Long?): Long {
         return i<!UNSAFE_CALL!>--<!> + i
     }
     if (i++ == 7L) {
-        return i++ <!NONE_APPLICABLE!>+<!> i
+        return i++ <!UNSAFE_OPERATOR_CALL!>+<!> i
     }
     return 0L
 }