Fix reflective access on overridden generic property reference

Generation of callable reference's signature in codegen should use the same
mechanism for obtaining the signature as the runtime in RuntimeTypeMapper,
namely DescriptorUtils.unwrapFakeOverride(...).original

 #KT-13700 Fixed

compiler/testData/codegen/box/reflection/functions/genericOverriddenFunction.kt

@@ -0,0 +1,16 @@
+// WITH_REFLECT
+
+import kotlin.test.assertEquals
+
+interface H<T> {
+    fun foo(): T?
+}
+
+interface A : H<A>
+
+fun box(): String {
+    assertEquals("A?", A::foo.returnType.toString())
+    assertEquals("T?", H<A>::foo.returnType.toString())
+
+    return "OK"
+}