[FIR] Fix resolving of refs with implicit this in lambdas in smartcasts

compiler/testData/diagnostics/tests/smartCasts/implicitThisOnRefInLambdaInSmartcast.fir.kt

@@ -0,0 +1,7 @@
+fun Any.test() {
+    val x: () -> Int = when (this) {
+        is String -> { { length  } }
+        else -> { { 1 } }
+    }
+    <!UNRESOLVED_REFERENCE!>length<!>
+}