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/InfixCallNullability.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !WITH_NEW_INFERENCE
 class A() {
     operator infix fun plus(i : Int) {}