Add a test for the SIL type equality assert failures due to opaque return types

This is the test part of #70865 which was fixed by the commit 8d649a2

Author: hjyamauchi

test/SIL/Serialization/Inputs/opaque_return_type_equality_input.swift

@@ -0,0 +1,26 @@
+public protocol Reducer<State> {
+  associatedtype State
+}
+
+public class WindowData {}
+
+public struct CR<State, R: Reducer>: Reducer
+where State == R.State {
+}
+
+internal struct SidebarContextMenu<WindowState: WindowData>: Reducer {
+  typealias State = WindowState
+}
+
+public protocol Factory {
+  associatedtype X: Reducer<WindowData>
+  func build() -> X
+}
+
+public struct MyFactory<WindowState: WindowData>: Factory {
+  public typealias State = WindowData
+  public init() {}
+  public func build() -> some Reducer<WindowData> {
+    CR<WindowData, SidebarContextMenu<WindowData>>()
+  }
+}

test/SIL/Serialization/opaque_return_type_equality.swift

@@ -0,0 +1,13 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -swift-version 5 -disable-availability-checking -c %S/Inputs/opaque_return_type_equality_input.swift -emit-module-path %t/opaque_return_type_equality_input.swiftmodule -I %t -O -module-name opaque_return_type_equality_input -o %t/opaque_return_type_equality_input.o
+// RUN: %target-swift-frontend -swift-version 5 -disable-availability-checking -c %S/opaque_return_type_equality.swift -emit-module-path %t/opaque_return_type_equality.swiftmodule -I %t -O -module-name opaque_return_type_equality -o %t/opaque_return_type_equality.o
+
+// Check that the SIL type equality check asserts don't fail between the opaque return types and their underlying types.
+
+import opaque_return_type_equality_input
+
+public func build<F: Factory>(f: F) -> any Reducer<WindowData> {
+  return f.build()
+}
+
+build(f: MyFactory())