[swiftc (125 vs. 5184)] Add crasher in swift::ASTVisitor

Add test case for crash triggered in `swift::ASTVisitor`.

Current number of unresolved compiler crashers: 125 (5184 resolved)

Stack trace:

```
))#0 0x00000000031d2438 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x31d2438)
#1 0x00000000031d2c86 SignalHandler(int) (/path/to/swift/bin/swift+0x31d2c86)
#2 0x00007f4a6f296330 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x10330)
#3 0x00007f4a6da54c37 gsignal /build/eglibc-oGUzwX/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56:0
#4 0x00007f4a6da58028 abort /build/eglibc-oGUzwX/eglibc-2.19/stdlib/abort.c:91:0
#5 0x0000000000d57ff5 (anonymous namespace)::Verifier::walkToStmtPost(swift::Stmt*) (/path/to/swift/bin/swift+0xd57ff5)
#6 0x0000000000d68010 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xd68010)
#7 0x0000000000d6692e (anonymous namespace)::Traversal::doIt(swift::Decl*) (/path/to/swift/bin/swift+0xd6692e)
#8 0x0000000000d66544 swift::Decl::walk(swift::ASTWalker&) (/path/to/swift/bin/swift+0xd66544)
#9 0x0000000000dbbfae swift::SourceFile::walk(swift::ASTWalker&) (/path/to/swift/bin/swift+0xdbbfae)
#10 0x0000000000d4e13c swift::verify(swift::SourceFile&) (/path/to/swift/bin/swift+0xd4e13c)
#11 0x0000000000afc803 swift::Parser::parseTopLevel() (/path/to/swift/bin/swift+0xafc803)
#12 0x0000000000b33b00 swift::parseIntoSourceFile(swift::SourceFile&, unsigned int, bool*, swift::SILParserState*, swift::PersistentParserState*, swift::DelayedParsingCallbacks*) (/path/to/swift/bin/swift+0xb33b00)
#13 0x0000000000938c43 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x938c43)
#14 0x000000000047ece5 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47ece5)
#15 0x000000000047db7f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47db7f)
#16 0x000000000044509a main (/path/to/swift/bin/swift+0x44509a)
#17 0x00007f4a6da3ff45 __libc_start_main /build/eglibc-oGUzwX/eglibc-2.19/csu/libc-start.c:321:0
#18 0x0000000000442816 _start (/path/to/swift/bin/swift+0x442816)
```

validation-test/compiler_crashers/28498-anonymous-namespace-verifier-walktostmtpost-swift-stmt.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 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
+#if8
+if let x