FirClassAnySynthesizedMemberScope.kt: make it a declared member scope

Related to KT-54844, KT-58926

compiler/testData/codegen/box/fir/delegatedAndDataTogether.kt

@@ -0,0 +1,20 @@
+// TARGET_BACKEND: JVM_IR
+// ISSUE: KT-58926
+
+fun box(): String {
+    val i1 = Impl()
+    val i2 = Impl()
+    val d1 = Data(i1, 1)
+    val d2 = Data(i2, 2)
+    return if (d1 != d2) "FAIL: should be equal" else "OK"
+}
+
+interface AnyNeighbor {
+    override fun equals(other: Any?): Boolean
+}
+
+class Impl : AnyNeighbor {
+    override fun equals(other: Any?): Boolean = true
+}
+
+data class Data(val i: Impl, val j: Int) : AnyNeighbor by i