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/resolve/specialConstructions/elvisAsCall.fir.kt

@@ -23,13 +23,13 @@ fun testDataFlowInfo1(a: Int?, b: Int?) {
     val c: Int = a ?: b!!
     doInt(c)
     // b is nullable if a != null
-    b <!NONE_APPLICABLE!>+<!> 1
+    b <!UNSAFE_OPERATOR_CALL!>+<!> 1
 }
 
 fun testDataFlowInfo2(a: Int?, b: Int?) {
     doInt(a ?: b!!)
     // b is nullable if a != null
-    b <!NONE_APPLICABLE!>+<!> 1
+    b <!UNSAFE_OPERATOR_CALL!>+<!> 1
 }
 
 fun testTypeMismatch(a: String?, b: Any) {