[swiftc] Add 💥 case (😢 → 45, 😀 → 5089) triggered in swift::DiagnosticEngine::emitDiagnostic(…)

Add crash case with stack trace:

```
swift: /path/to/llvm/include/llvm/Support/Casting.h:95: static bool llvm::isa_impl_cl<swift::ParenType, const swift::TypeBase *>::doit(const From *) [To = swift::ParenType, From = const swift::TypeBase *]: Assertion `Val && "isa<> used on a null pointer"' failed.
10 swift           0x00000000010c6e1b swift::DiagnosticEngine::emitDiagnostic(swift::Diagnostic const&) + 2923
11 swift           0x00000000010c608f swift::DiagnosticEngine::flushActiveDiagnostic() + 319
12 swift           0x0000000000e9c6e7 swift::TypeChecker::validateDecl(swift::ValueDecl*, bool) + 2391
19 swift           0x0000000000ea1586 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
20 swift           0x0000000000ec3732 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 1026
21 swift           0x0000000000c59299 swift::CompilerInstance::performSema() + 3289
23 swift           0x00000000007d72b9 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2857
24 swift           0x00000000007a32c8 main + 2872
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28324-swift-diagnosticengine-emitdiagnostic.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28324-swift-diagnosticengine-emitdiagnostic-23bd53.o
1.	While type-checking 'a' at validation-test/compiler_crashers/28324-swift-diagnosticengine-emitdiagnostic.swift:10:1
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```

Author: practicalswift

validation-test/compiler_crashers/28324-swift-diagnosticengine-emitdiagnostic.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 -parse
+// REQUIRES: asserts
+protocol a{extension{@objc protocol A:a