Add two regression tests for bugs already fixed

Author: slavapestov

test/decl/protocol/req/issue-10831.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift
+
+struct G<T> {}
+  
+protocol P {
+  associatedtype T
+  associatedtype U
+}
+
+protocol Q: P where T == G<U> {}
+
+protocol R: Q where U == Int {}
+
+struct X: R {}

test/decl/protocol/req/issue-12943.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Composite<Foo> { }
+
+protocol Chain {
+  associatedtype Next: Chain
+  associatedtype Foo = Composite<Next.Foo>
+}
+
+// expected-error@+3 {{cannot build rewrite system for protocol; rule length limit exceeded}}
+// expected-note@+2 {{failed rewrite rule}}
+// expected-error@+1 {{'Foo' is not a member type of type 'Self.Next'}}
+protocol OtherChain where Foo == Composite<Next.Foo> {
+  associatedtype Next: OtherChain
+  associatedtype Foo
+}