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/tpInBounds.fir.kt

@@ -8,7 +8,7 @@ class A<T : CharSequence?, E1 : T, E2: T?> {
         x.bar()
 
         if (1 == 1) {
-            y.<!INAPPLICABLE_CANDIDATE!>bar<!>()
+            y<!UNSAFE_CALL!>.<!>bar()
         }
 
         x?.bar()