[swiftc (52 vs. 5407)] Add crasher in ?

Add test case for crash triggered in `?`.

Current number of unresolved compiler crashers: 52 (5407 resolved)

Stack trace:

```
0 0x00000000035247a8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x35247a8)
1 0x0000000003524ee6 SignalHandler(int) (/path/to/swift/bin/swift+0x3524ee6)
2 0x00007fc1555d03e0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x113e0)
3 0x00007fc153f36428 gsignal /build/glibc-Qz8a69/glibc-2.23/signal/../sysdeps/unix/sysv/linux/raise.c:54:0
4 0x00007fc153f3802a abort /build/glibc-Qz8a69/glibc-2.23/stdlib/abort.c:91:0
5 0x00000000034c057d llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) (/path/to/swift/bin/swift+0x34c057d)
6 0x0000000000df5ace swift::TypeVisitor<(anonymous namespace)::TypePrinter, void>::visit(swift::Type) (/path/to/swift/bin/swift+0xdf5ace)
7 0x0000000000de3654 (anonymous namespace)::TypePrinter::visit(swift::Type) (/path/to/swift/bin/swift+0xde3654)
8 0x0000000000de3591 swift::Type::print(llvm::raw_ostream&, swift::PrintOptions const&) const (/path/to/swift/bin/swift+0xde3591)
9 0x0000000000dccd32 (anonymous namespace)::PrintDecl::printParameter(swift::ParamDecl const*) (/path/to/swift/bin/swift+0xdccd32)
10 0x0000000000db7c54 swift::Decl::dump(llvm::raw_ostream&, unsigned int) const (/path/to/swift/bin/swift+0xdb7c54)
11 0x0000000000e139c9 (anonymous namespace)::Verifier::checkErrors(swift::ValueDecl*) (/path/to/swift/bin/swift+0xe139c9)
12 0x0000000000e08e22 (anonymous namespace)::Verifier::walkToDeclPost(swift::Decl*) (/path/to/swift/bin/swift+0xe08e22)
13 0x0000000000e17a79 (anonymous namespace)::Traversal::doIt(swift::Decl*) (/path/to/swift/bin/swift+0xe17a79)
14 0x0000000000e1a06e (anonymous namespace)::Traversal::visitParameterList(swift::ParameterList*) (/path/to/swift/bin/swift+0xe1a06e)
15 0x0000000000e183f8 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) (/path/to/swift/bin/swift+0xe183f8)
16 0x0000000000e19fc7 (anonymous namespace)::Traversal::visitCollectionExpr(swift::CollectionExpr*) (/path/to/swift/bin/swift+0xe19fc7)
17 0x0000000000e18252 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) (/path/to/swift/bin/swift+0xe18252)
18 0x0000000000e1a26e (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) (/path/to/swift/bin/swift+0xe1a26e)
19 0x0000000000e1a584 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xe1a584)
20 0x0000000000e1761d (anonymous namespace)::Traversal::doIt(swift::Decl*) (/path/to/swift/bin/swift+0xe1761d)
21 0x0000000000e17394 swift::Decl::walk(swift::ASTWalker&) (/path/to/swift/bin/swift+0xe17394)
22 0x0000000000e7281e swift::SourceFile::walk(swift::ASTWalker&) (/path/to/swift/bin/swift+0xe7281e)
23 0x0000000000dff575 swift::verify(swift::SourceFile&) (/path/to/swift/bin/swift+0xdff575)
24 0x0000000000c25409 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) (/path/to/swift/bin/swift+0xc25409)
25 0x00000000009998d6 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x9998d6)
26 0x000000000047ca79 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47ca79)
27 0x000000000043b2b7 main (/path/to/swift/bin/swift+0x43b2b7)
28 0x00007fc153f21830 __libc_start_main /build/glibc-Qz8a69/glibc-2.23/csu/../csu/libc-start.c:325:0
29 0x00000000004386f9 _start (/path/to/swift/bin/swift+0x4386f9)
```

Author: practicalswift

validation-test/compiler_crashers/28647-unreachable-executed-at-swift-include-swift-ast-typevisitor-h-39.swift

@@ -0,0 +1,11 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 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
+
+// REQUIRES: deterministic-behavior
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+c|[{t in(0/0