[swiftc (34 vs. 5390)] Add crasher in swift::Parser::classifyConditionalCompilationExpr

Add test case for crash triggered in `swift::Parser::classifyConditionalCompilationExpr`.

Current number of unresolved compiler crashers: 34 (5390 resolved)

Assertion failure in `llvm/include/llvm/Support/Casting.h (line 237)`:

```
Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

When executing: typename cast_retty<X, Y *>::ret_type llvm::cast(Y *) [X = swift::UnresolvedDeclRefExpr, Y = swift::Expr]
```

Assertion context:

```
                          typename simplify_type<Y>::SimpleType>::doit(Val);
}

template <class X, class Y>
inline typename cast_retty<X, Y *>::ret_type cast(Y *Val) {
  assert(isa<X>(Val) && "cast<Ty>() argument of incompatible type!");
  return cast_convert_val<X, Y*,
                          typename simplify_type<Y*>::SimpleType>::doit(Val);
}

// cast_or_null<X> - Functionally identical to cast, except that a null value is
```
Stack trace:

```
0 0x0000000003515068 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x3515068)
1 0x00000000035157a6 SignalHandler(int) (/path/to/swift/bin/swift+0x35157a6)
2 0x00007f3a574ea3e0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x113e0)
3 0x00007f3a55e50428 gsignal /build/glibc-Qz8a69/glibc-2.23/signal/../sysdeps/unix/sysv/linux/raise.c:54:0
4 0x00007f3a55e5202a abort /build/glibc-Qz8a69/glibc-2.23/stdlib/abort.c:91:0
5 0x00007f3a55e48bd7 __assert_fail_base /build/glibc-Qz8a69/glibc-2.23/assert/assert.c:92:0
6 0x00007f3a55e48c82 (/lib/x86_64-linux-gnu/libc.so.6+0x2dc82)
7 0x0000000000bf198d swift::Parser::classifyConditionalCompilationExpr(swift::Expr*, swift::ASTContext&, swift::DiagnosticEngine&, bool) (/path/to/swift/bin/swift+0xbf198d)
8 0x0000000000bea8b2 swift::Parser::parseStmtIfConfig(swift::BraceItemListKind) (/path/to/swift/bin/swift+0xbea8b2)
9 0x0000000000be9904 swift::Parser::parseBraceItems(llvm::SmallVectorImpl<swift::ASTNode>&, swift::BraceItemListKind, swift::BraceItemListKind) (/path/to/swift/bin/swift+0xbe9904)
10 0x0000000000b82cf6 swift::Parser::parseTopLevel() (/path/to/swift/bin/swift+0xb82cf6)
11 0x0000000000bb71d0 swift::parseIntoSourceFile(swift::SourceFile&, unsigned int, bool*, swift::SILParserState*, swift::PersistentParserState*, swift::DelayedParsingCallbacks*) (/path/to/swift/bin/swift+0xbb71d0)
12 0x0000000000998d33 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x998d33)
13 0x000000000047ca5a swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47ca5a)
14 0x000000000043b297 main (/path/to/swift/bin/swift+0x43b297)
15 0x00007f3a55e3b830 __libc_start_main /build/glibc-Qz8a69/glibc-2.23/csu/../csu/libc-start.c:325:0
16 0x00000000004386d9 _start (/path/to/swift/bin/swift+0x4386d9)
```

Author: practicalswift

validation-test/compiler_crashers/28611-isa-x-val-cast-ty-argument-of-incompatible-type.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
+#if0=0