Add a regression test for https://bugs.swift.org/browse/SR-15790

Author: slavapestov

test/Generics/sr15790.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=on 2>&1 | %FileCheck %s
+
+// CHECK-LABEL: sr15790.(file).P1@
+// CHECK-NEXT: Requirement signature: <Self where Self.[P1]X : P1, Self.[P1]X == Self.[P1]X.[P1]X>
+public protocol P1 {
+  associatedtype X: P1 where X.X == X
+}
+
+// CHECK-LABEL: sr15790.(file).P2@
+// CHECK-NEXT: Requirement signature: <Self where Self : Collection, Self : P1, Self.[Sequence]Element : P1, Self.[P1]X == Self.[P2]Z.[P2]Y, Self.[P2]Y : P3, Self.[P2]Z : P2>
+public protocol P2: Collection, P1 where Element: P1, X == Z.Y {
+  associatedtype Y: P3
+  associatedtype Z: P2
+}
+
+// CHECK-LABEL: sr15790.(file).P3@
+// CHECK-NEXT: Requirement signature: <Self where Self : P2, Self.[P3]T : P2>
+public protocol P3: P2 {
+  associatedtype T: P2
+}