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/smartCasts.fir.kt

@@ -24,7 +24,7 @@ fun <T : CharSequence?> foo(x: T) {
         x<!UNNECESSARY_SAFE_CALL!>?.<!>bar1()
     }
 
-    x.<!INAPPLICABLE_CANDIDATE!>length<!>
+    x<!UNSAFE_CALL!>.<!>length
 
     if (x is String) {
         x.length