[FE 1.0] Add fallback for extracting when call

compiler/testData/diagnostics/tests/when/extractingEntireCall.kt

@@ -0,0 +1,15 @@
+// WITH_STDLIB
+
+interface FirExpression
+interface FirNamedArgumentExpression : FirExpression {
+    val expression: FirNamedArgumentExpression
+}
+class AnnotationUseSiteTarget
+
+private fun List<FirExpression>.toAnnotationUseSiteTargets2(): Set<AnnotationUseSiteTarget> =
+    flatMapTo(mutableSetOf()) { arg ->
+        when (val unwrappedArg = if (arg is FirNamedArgumentExpression) <!DEBUG_INFO_SMARTCAST!>arg<!>.expression else arg) {
+        is FirNamedArgumentExpression -> setOf()
+        else -> setOf()
+    }
+    }