Add regression test for https://bugs.swift.org/browse/SR-12691

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr12691.swift

@@ -0,0 +1,23 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+struct CountSteps1<T> : Collection {
+  init(count: Int) { self.count = count }
+  var count: Int
+
+  var startIndex: Int { 0 }
+  var endIndex: Int { count }
+  func index(after i: Int) -> Int { 
+    totalSteps += 1
+    return i + 1
+  }
+  subscript(i: Int) -> Int { return i }
+}
+
+extension CountSteps1
+  : RandomAccessCollection, BidirectionalCollection 
+     where T : Equatable 
+{
+  func index(_ i: Index, offsetBy d: Int) -> Index {
+    return i + d
+  }
+}