FIR: add UnsafeCall resolution diagnostics

Previously unsafe call is reported as part of InapplicableWrongReceiver.
This makes it difficult for the downstream checkers to report different
diagnostics.

compiler/testData/diagnostics/tests/generics/nullability/smartCastRefinedClass.fir.kt

@@ -1,6 +1,6 @@
 fun <T : Any?> foo(x: T) {
     if (x is String?) {
-        x.<!INAPPLICABLE_CANDIDATE!>length<!>
+        x<!UNSAFE_CALL!>.<!>length
 
         if (x != null) {
             x.length