[swiftc] Add test case for crash triggered in swift::TypeChecker::checkGenericParamList(swift::ArchetypeBuilder*, swift::GenericParamList*, swift::GenericSignature*, bool, swift::GenericTypeResolver*)

Stack trace:

```
4  swift           0x0000000000ed9e0a swift::TypeChecker::checkGenericParamList(swift::ArchetypeBuilder*, swift::GenericParamList*, swift::GenericSignature*, bool, swift::GenericTypeResolver*) + 74
5  swift           0x0000000000edb637 swift::TypeChecker::validateGenericSignature(swift::GenericParamList*, swift::DeclContext*, swift::GenericSignature*, std::function<bool (swift::ArchetypeBuilder&)>, bool&) + 135
6  swift           0x0000000000edb9d6 swift::TypeChecker::validateGenericTypeSignature(swift::GenericTypeDecl*) + 102
7  swift           0x0000000000e9c94f swift::TypeChecker::validateDecl(swift::ValueDecl*, bool) + 367
12 swift           0x0000000000ea2166 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
13 swift           0x0000000000ec4ac2 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 1026
14 swift           0x0000000000c58d39 swift::CompilerInstance::performSema() + 3289
16 swift           0x00000000007d73bf swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2863
17 swift           0x00000000007a33d8 main + 2872
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28308-swift-typechecker-checkgenericparamlist.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28308-swift-typechecker-checkgenericparamlist-b4f9c8.o
1.	While type-checking 'B' at validation-test/compiler_crashers/28308-swift-typechecker-checkgenericparamlist.swift:9:1
<unknown>:0: error: unable to execute command: Segmentation fault
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```

Author: practicalswift

validation-test/compiler_crashers/28308-swift-typechecker-checkgenericparamlist.swift

@@ -0,0 +1,71 @@
+// 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
+struct B<b where T>{{return"class A<b protocol b{
+func<b {
+func g:BooleanType
+struct B{
+class A.
+struct B{n C{struct B:A<d where f={
+{}struct b: A.h: T where g:b{let l=c<d where B{class B<T where k:a()class A{let:A{
+class B{
+{
+{
+let a{
+if true {class c
+class B:a{
+let a{let e{let e=b
+{
+struct c {
+if{
+class b
+struct B:e
+if true {}
+{
+{func a(){{let l=b
+import Foundation
+if{
+enum S<T where k:A{
+var f{let a=b where B<T : A{
+import Foundation
+class A.h:BooleanType
+func c
+{
+let t:b)class g:e
+func a{struct S{class c<T where B:a{let:A{
+class B{let a {n C{class B<T where k:b)class a(){
+func a{func c{
+if true {let a(){n C{var f.
+{}
+import c<T:A.h:b
+enum S<b where T:b{
+{
+class a{
+struct g {class A{
+{
+class A{
+func c
+let a=b where f={{class B<h
+{let a={
+var f=b){class b
+{e f=i[where[{struct B<b
+{struct
+if true{struct B<T where h:a{}
+class b{let a{
+enum S{
+var f.
+class B<T where T:d
+class A{
+{
+if true {
+{
+class g:a
+import c
+class B<T where g:d
+typealias F=bhere k:A{return"class B<b
+enum b where B:BooleanType