[FIR] Unify code of type unification, fix intersection types processing in FirCastDiagnosticsHelpers.kt

compiler/testData/diagnostics/tests/smartCasts/kt10444.kt

@@ -1,6 +1,7 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 // KT-10444 Do not ignore smart (unchecked) casts to the same classifier
 
+class Base<in T>
 class Qwe<T : Any>(val a: T?) {
     fun test1(obj: Any) {
         obj <!UNCHECKED_CAST!>as Qwe<T><!>
@@ -12,6 +13,10 @@ class Qwe<T : Any>(val a: T?) {
         check(obj.a)
     }
 
+    fun test2(b: Base<*>) {
+        b <!UNCHECKED_CAST!>as Base<Any><!>
+    }
+
     fun check(a: T?) {
     }
 }