[swiftc (119 vs. 5184)] Add crasher in swift::addTrivialAccessorsToSt…

…orage

Add test case for crash triggered in `swift::addTrivialAccessorsToStorage`.

Current number of unresolved compiler crashers: 119 (5184 resolved)

Assertion failure in [`lib/Sema/CodeSynthesis.cpp (line 43)`](https://github.com/apple/swift/blob/master/lib/Sema/CodeSynthesis.cpp#L43):

```
Assertion `(isa<AbstractFunctionDecl>(DC) || isa<FileUnit>(DC)) && "Unknown declcontext"' failed.

When executing: void addMemberToContextIfNeeded(swift::Decl *, swift::DeclContext *, swift::Decl *)
```

Assertion context:

```
    ntd->addMember(D, Hint);
  } else if (auto *ed = dyn_cast<ExtensionDecl>(DC)) {
    ed->addMember(D, Hint);
  } else {
    assert((isa<AbstractFunctionDecl>(DC) || isa<FileUnit>(DC)) &&
           "Unknown declcontext");
  }
}

static ParamDecl *getParamDeclAtIndex(FuncDecl *fn, unsigned index) {
  return fn->getParameterLists().back()->get(index);
```
Stack trace:

```
#0 0x00000000031d10e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x31d10e8)
#1 0x00000000031d1936 SignalHandler(int) (/path/to/swift/bin/swift+0x31d1936)
#2 0x00007f866fd58330 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x10330)
#3 0x00007f866e516c37 gsignal /build/eglibc-oGUzwX/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56:0
#4 0x00007f866e51a028 abort /build/eglibc-oGUzwX/eglibc-2.19/stdlib/abort.c:91:0
#5 0x00007f866e50fbf6 __assert_fail_base /build/eglibc-oGUzwX/eglibc-2.19/assert/assert.c:92:0
#6 0x00007f866e50fca2 (/lib/x86_64-linux-gnu/libc.so.6+0x2fca2)
#7 0x0000000000c991a0 swift::addTrivialAccessorsToStorage(swift::AbstractStorageDecl*, swift::TypeChecker&) (/path/to/swift/bin/swift+0xc991a0)
#8 0x0000000000c9a853 swift::maybeAddAccessorsToVariable(swift::VarDecl*, swift::TypeChecker&) (/path/to/swift/bin/swift+0xc9a853)
#9 0x0000000000b9c1a2 swift::TypeChecker::validateDecl(swift::ValueDecl*, bool) (/path/to/swift/bin/swift+0xb9c1a2)
#10 0x0000000000bb6157 std::_Function_handler<void (swift::VarDecl*), (anonymous namespace)::DeclChecker::visitBoundVars(swift::Pattern*)::{lambda(swift::VarDecl*)#1}>::_M_invoke(std::_Any_data const&, swift::VarDecl*) (/path/to/swift/bin/swift+0xbb6157)
#11 0x0000000000dcdcf8 swift::Pattern::forEachVariable(std::function<void (swift::VarDecl*)> const&) const (/path/to/swift/bin/swift+0xdcdcf8)
#12 0x0000000000ba1fb1 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0xba1fb1)
#13 0x0000000000ba1d06 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) (/path/to/swift/bin/swift+0xba1d06)
#14 0x0000000000c0253f swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xc0253f)
#15 0x0000000000c01665 (anonymous namespace)::StmtChecker::typeCheckBody(swift::BraceStmt*&) (/path/to/swift/bin/swift+0xc01665)
#16 0x0000000000c01c16 swift::TypeChecker::typeCheckClosureBody(swift::ClosureExpr*) (/path/to/swift/bin/swift+0xc01c16)
#17 0x0000000000c1fc7c swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool, bool) (/path/to/swift/bin/swift+0xc1fc7c)
#18 0x0000000000b8da04 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) (/path/to/swift/bin/swift+0xb8da04)
#19 0x0000000000c02594 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xc02594)
#20 0x0000000000c01db6 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) (/path/to/swift/bin/swift+0xc01db6)
#21 0x0000000000c15d0a swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) (/path/to/swift/bin/swift+0xc15d0a)
#22 0x0000000000938c66 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x938c66)
#23 0x000000000047ece5 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47ece5)
#24 0x000000000047db7f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47db7f)
#25 0x000000000044509a main (/path/to/swift/bin/swift+0x44509a)
#26 0x00007f866e501f45 __libc_start_main /build/eglibc-oGUzwX/eglibc-2.19/csu/libc-start.c:321:0
#27 0x0000000000442816 _start (/path/to/swift/bin/swift+0x442816)
```

validation-test/compiler_crashers/28492-isa-abstractfunctiondecl-dc-isa-fileunit-dc-unknown-declcontext.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 -emit-ir
+// REQUIRES: asserts
+{lazy var E