Merge pull request #5766 from practicalswift/swiftc-28508-unreachable-executed-at-swift-lib-sema-csgen-cpp-2656

[swiftc (133 vs. 5186)] Add crasher in swift::ASTVisitor

Author: practicalswift

Diff for: validation-test/compiler_crashers/28508-unreachable-executed-at-swift-lib-sema-csgen-cpp-2656.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 A{{}func a{}struct A{lazy var f=a