Add regression test for https://bugs.swift.org/browse/SR-13141

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr13141.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public protocol Book {
+  associatedtype Name
+}
+public protocol BookDecorator: Book where Name == DecoratedBook.Name {
+  associatedtype DecoratedBook: Book
+  associatedtype Name = DecoratedBook.Name
+}
+public class ConcreteBookDecorator<T: Book>: BookDecorator {
+  public typealias DecoratedBook = T
+}