Add a regression test for one of the crashes from https://bugs.swift.org/browse/SR-15792

Author: slavapestov

test/Generics/sr15792_1.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift %s
+
+protocol Collection {
+  associatedtype SubSequence: Collection
+}
+
+protocol BidirectionalCollection: Collection where SubSequence: BidirectionalCollection {}
+
+struct Slice<Base : Collection> : Collection {
+  typealias SubSequence = Slice<Base>
+}
+
+extension Slice: BidirectionalCollection where Base : BidirectionalCollection {}
+
+protocol SlicedCollection: BidirectionalCollection where SubSequence == Slice<Self> {}