[swiftc (88 vs. 5180)] Add crasher: UNREACHABLE executed at swift/include/swift/AST/CanTypeVisitor.h:41

Add test case for crash triggered in `?`.

Current number of unresolved compiler crashers: 88 (5180 resolved)

Stack trace:

```
0  swift           0x00000000031da6c8
1  swift           0x00000000031daf46
2  libpthread.so.0 0x00007ff86e2b5330
3  libc.so.6       0x00007ff86ca73c37 gsignal + 55
4  libc.so.6       0x00007ff86ca77028 abort + 328
5  swift           0x000000000318311d
6  swift           0x0000000000c74998
7  swift           0x0000000000c73156
8  swift           0x0000000000c72bfe
9  swift           0x0000000000c6fda2
10 swift           0x0000000000c6e237
11 swift           0x0000000000c69d0f
12 swift           0x0000000000c5a9a6
13 swift           0x0000000000c60e90
14 swift           0x0000000000ba991f
15 swift           0x0000000000bac3be
16 swift           0x0000000000bafe77
17 swift           0x0000000000c2099d
18 swift           0x0000000000c202bc
19 swift           0x0000000000c1f455
20 swift           0x0000000000c1e7a3
21 swift           0x0000000000c1e5f7
22 swift           0x0000000000c1f1fc
23 swift           0x0000000000c32e18
24 swift           0x0000000000c3398b
25 swift           0x0000000000957a26
26 swift           0x00000000004a050e
27 swift           0x00000000004674de
28 libc.so.6       0x00007ff86ca5ef45 __libc_start_main + 245
29 swift           0x0000000000464be6
```

Author: practicalswift

validation-test/compiler_crashers/28457-unreachable-executed-at-swift-include-swift-ast-cantypevisitor-h-41.swift

@@ -0,0 +1,11 @@
+// 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
+let a{
+for
+let a{String($0}({