Fix AssertionError in CodegenUtil.getDelegates

Change getAllOverriddenDescriptors contracti, now it returns
original (not substituted) descriptors without any duplicates.

First of all it's necessary in CodegenUtil.getDelegates to avoid
duplicates (see assertion there), but also it's convenient for all
other usages of this method

 #KT-8154 Fixed

compiler/testData/codegen/box/delegation/kt8154.kt

@@ -0,0 +1,19 @@
+interface A<T> {
+    fun foo(): T
+}
+
+interface B<T> : A<T>
+
+class BImpl<T>(a: A<T>) : B<T>, A<T> by a
+
+fun box(): String {
+    val b: B<String> = BImpl(object : A<String> {
+        override fun foo() = "OK"
+    })
+
+    if (b.foo() != "OK") return "fail 1"
+
+    val a: A<String> = b
+
+    return a.foo()
+}