[swiftc (25 vs. 5511)] Add crasher in swift::TypeBase::getContextSubstitutions

Add test case for crash triggered in `swift::TypeBase::getContextSubstitutions`.

Current number of unresolved compiler crashers: 25 (5511 resolved)

Stack trace:

```
0 0x000000000395df08 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x395df08)
1 0x000000000395e646 SignalHandler(int) (/path/to/swift/bin/swift+0x395e646)
2 0x00007f43e1b53390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
3 0x00007f43e0079428 gsignal /build/glibc-9tT8Do/glibc-2.23/signal/../sysdeps/unix/sysv/linux/raise.c:54:0
4 0x00007f43e007b02a abort /build/glibc-9tT8Do/glibc-2.23/stdlib/abort.c:91:0
5 0x00000000038fa2dd llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) (/path/to/swift/bin/swift+0x38fa2dd)
6 0x00000000014f5458 swift::TypeBase::getContextSubstitutions(swift::DeclContext const*, swift::GenericEnvironment*) (/path/to/swift/bin/swift+0x14f5458)
7 0x00000000014f0f00 swift::TypeBase::getContextSubstitutionMap(swift::ModuleDecl*, swift::DeclContext const*, swift::GenericEnvironment*) (/path/to/swift/bin/swift+0x14f0f00)
8 0x00000000013158eb swift::TypeChecker::substMemberTypeWithBase(swift::ModuleDecl*, swift::TypeDecl*, swift::Type) (/path/to/swift/bin/swift+0x13158eb)
9 0x00000000012d961d swift::TypeChecker::lookupMemberType(swift::DeclContext*, swift::Type, swift::Identifier, swift::OptionSet<swift::NameLookupFlags, unsigned int>) (/path/to/swift/bin/swift+0x12d961d)
10 0x00000000013174bc resolveIdentTypeComponent(swift::TypeChecker&, swift::DeclContext*, llvm::ArrayRef<swift::ComponentIdentTypeRepr*>, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) (/path/to/swift/bin/swift+0x13174bc)
11 0x0000000001316cb3 swift::TypeChecker::resolveIdentifierType(swift::DeclContext*, swift::IdentTypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, bool, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) (/path/to/swift/bin/swift+0x1316cb3)
12 0x0000000001317dc7 (anonymous namespace)::TypeResolver::resolveType(swift::TypeRepr*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>) (/path/to/swift/bin/swift+0x1317dc7)
13 0x0000000001317ccc swift::TypeChecker::resolveType(swift::TypeRepr*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) (/path/to/swift/bin/swift+0x1317ccc)
14 0x00000000013163ca swift::TypeChecker::validateType(swift::TypeLoc&, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, llvm::function_ref<bool (swift::TypeCheckRequest)>*) (/path/to/swift/bin/swift+0x13163ca)
15 0x00000000012dc85a swift::TypeChecker::typeCheckPattern(swift::Pattern*, swift::DeclContext*, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>) (/path/to/swift/bin/swift+0x12dc85a)
16 0x00000000012ac2fc validatePatternBindingEntries(swift::TypeChecker&, swift::PatternBindingDecl*) (/path/to/swift/bin/swift+0x12ac2fc)
17 0x00000000012a6824 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x12a6824)
18 0x00000000012a66e3 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) (/path/to/swift/bin/swift+0x12a66e3)
19 0x0000000001310516 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0x1310516)
20 0x000000000130fd86 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) (/path/to/swift/bin/swift+0x130fd86)
21 0x0000000001324890 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) (/path/to/swift/bin/swift+0x1324890)
22 0x0000000000f950f6 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0xf950f6)
23 0x00000000004a7d6b swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x4a7d6b)
24 0x00000000004650b7 main (/path/to/swift/bin/swift+0x4650b7)
25 0x00007f43e0064830 __libc_start_main /build/glibc-9tT8Do/glibc-2.23/csu/../csu/libc-start.c:325:0
26 0x0000000000462759 _start (/path/to/swift/bin/swift+0x462759)
```

Author: practicalswift

validation-test/compiler_crashers/28726-nominaltypedecl-hasfixedlayout.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
+// non-fuzz
+struct T<b{typealias a=(}let:T.a=