Merge pull request #6594 from practicalswift/swiftc-28612-val-isa-used-on-a-null-pointer

[swiftc (35 vs. 5390)] Add crasher in swift::ASTVisitor

Author: practicalswift

validation-test/compiler_crashers/28612-val-isa-used-on-a-null-pointer.swift

@@ -0,0 +1,10 @@
+// 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 https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+// REQUIRES: asserts
+switch{case{}|0/(let(0t