K2: Unwrap fake overrides before using them as DFA identifiers

It's necessary because even for stable a.b.c.d we can't guarantee that
this reference will always point to the same symbol because
different capture type instantiations generate different scopes
with different resulting symbol instances.

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartCastOnValueBoundToSubjectVariable.fir.kt

@@ -43,6 +43,6 @@ fun test5(x: Inv<out Any?>) {
 
 fun test6(x: Inv<out String?>) {
     when (val y = x.data) {
-        is String -> x.data<!UNSAFE_CALL!>.<!>length // should be ok
+        is String -> x.data.length // should be ok
     }
 }