Merge pull request #5750 from practicalswift/swiftc-28500-unreachable-executed-at-swift-lib-sema-typecheckdecl-cpp-1687

[swiftc (127 vs. 5184)] Add crasher in swift::TypeChecker::typeCheckDecl

Author: practicalswift

Diff for: validation-test/compiler_crashers/28500-unreachable-executed-at-swift-lib-sema-typecheckdecl-cpp-1687.swift

@@ -0,0 +1,9 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+protocol S{precedencegroup s{}