Add regression test for fixed crasher

Author: slavapestov

test/Generics/issue-85084.swift

@@ -0,0 +1,19 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P1 {}
+
+protocol P2 {
+    associatedtype U
+}
+
+protocol P3<T> {
+    associatedtype T: P2
+}
+
+struct S1<T: P2>: P3 where T.U: P1 {}
+
+extension P3 where Self == S1<T> {  // expected-error {{same-type constraint 'Self' == 'S1<Self.T>' is recursive}}
+    static var a: S1<T> {
+        S1<T>()
+    }
+}