[swiftc] Add 💥 case (😢 → 41, 😀 → 5090) triggered in swift::TypeChecker::checkConformance(…)

Add crash case with stack trace:

```
swift: /path/to/swift/lib/Sema/TypeCheckProtocol.cpp:1583: diag::RequirementKind getRequirementKind(swift::ValueDecl *): Assertion `isa<SubscriptDecl>(VD) && "Unhandled requirement kind"' failed.
10 swift           0x0000000000ee7bef swift::TypeChecker::checkConformance(swift::NormalProtocolConformance*) + 1615
12 swift           0x0000000000ec3671 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 1281
13 swift           0x0000000000c590f9 swift::CompilerInstance::performSema() + 3289
15 swift           0x00000000007d72b9 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2857
16 swift           0x00000000007a32c8 main + 2872
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28319-swift-typechecker-checkconformance.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28319-swift-typechecker-checkconformance-4d62c4.o
<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/28319-swift-typechecker-checkconformance.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{enum B:A{let:e var _=B}typealias e