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/nullabilityAndSmartCasts/NullableNothingIsExactlyNull.fir.kt

@@ -2,7 +2,7 @@ fun test() {
   val out : Int? = null
   val x : Nothing? = null
   if (out != x)
-    out.<!NONE_APPLICABLE!>plus<!>(1)
+    out<!UNSAFE_CALL!>.<!>plus(1)
   if (out == x) return
-  out.<!NONE_APPLICABLE!>plus<!>(1)
+  out<!UNSAFE_CALL!>.<!>plus(1)
 }