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/smartCastsOnThis.fir.kt

@@ -23,7 +23,7 @@ fun <T : String?> T.foo() {
         this<!UNNECESSARY_SAFE_CALL!>?.<!>bar1()
     }
 
-    <!INAPPLICABLE_CANDIDATE!>length<!>
+    <!UNSAFE_CALL!>length<!>
 
     if (this is String) {
         length