Issue Description:

public func headers() -> [AnyHashable: String]? { fatalError() }
public var httpAdditionalHeaders: [AnyHashable : Any]?

if let userAgentHeaders = headers() {
  httpAdditionalHeaders = userAgentHeaders #^COMPLETE^#
}

{code:title=$ swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename /tmp/test.swift}
found code completion token COMPLETE at offset 202
found code completion token COMPLETE at offset 202
Assertion failed: (value != OpenExistentials.end() && "didn't see this OVE in a containing OpenExistentialExpr?"), function walkToExprPre, file /Users/nathanhawes/workspace/open-swift-source-compat-suite-master/swift/lib/Sema/CSGen.cpp, line 3179.
0 swift-ide-test 0x0000000108d5f8b8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1 swift-ide-test 0x0000000108d5e826 llvm::sys::RunSignalHandlers() + 198
2 swift-ide-test 0x0000000108d5fed2 SignalHandler(int) + 258
3 libsystem_platform.dylib 0x00007fff7dd65f5a _sigtramp + 26
4 libsystem_platform.dylib 0x00007fb60381ba00 _sigtramp + 2242599616
5 libsystem_c.dylib 0x00007fff7db0f1ae abort + 127
6 libsystem_c.dylib 0x00007fff7dad71ac basename_r + 0
7 swift-ide-test 0x0000000105a4b290 (anonymous namespace)::SanitizeExpr::walkToExprPre(swift::Expr*) + 528
8 swift-ide-test 0x0000000105da6c28 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 4296
9 swift-ide-test 0x0000000105da5be6 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 134
10 swift-ide-test 0x0000000105da5be6 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 134
11 swift-ide-test 0x0000000105da7d7f (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 207
12 swift-ide-test 0x0000000105da5504 swift::Expr::walk(swift::ASTWalker&) + 84
13 swift-ide-test 0x0000000105a43011 swift::constraints::ConstraintSystem::generateConstraints(swift::Expr*) + 145
14 swift-ide-test 0x0000000105a7cae7 swift::constraints::ConstraintSystem::solveImpl(swift::Expr*&, swift::Type, swift::ExprTypeCheckListener*, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::FreeTypeVariableBinding) + 471
15 swift-ide-test 0x0000000105a7c68f swift::constraints::ConstraintSystem::solve(swift::Expr*&, swift::Type, swift::ExprTypeCheckListener*, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::FreeTypeVariableBinding) + 31
16 swift-ide-test 0x0000000105b168ea swift::TypeChecker::getTypeOfExpressionWithoutApplying(swift::Expr*&, swift::DeclContext*, swift::ConcreteDeclRef&, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*) + 234
17 swift-ide-test 0x0000000105bc5d48 getTypeOfCompletionContextExpr(swift::TypeChecker&, swift::DeclContext*, swift::CompletionTypeCheckKind, swift::Expr*&, swift::ConcreteDeclRef&) + 168
18 swift-ide-test 0x0000000105bc57f4 swift::getTypeOfCompletionContextExpr(swift::ASTContext&, swift::DeclContext*, swift::CompletionTypeCheckKind, swift::Expr*&, swift::ConcreteDeclRef&) + 84
19 swift-ide-test 0x00000001057efdde (anonymous namespace)::CodeCompletionCallbacksImpl::doneParsing() + 16702
20 swift-ide-test 0x0000000105ce3ff9 swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 377
21 swift-ide-test 0x00000001057d3fd9 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 1081
22 swift-ide-test 0x00000001057d3269 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 505
23 swift-ide-test 0x0000000105758212 main + 27554
24 libdyld.dylib 0x00007fff7da63015 start + 1
25 libdyld.dylib 0x0000000000000005 start + 2186924017
Stack dump:
0. Program arguments: ../open-swift-source-compat-suite-master/build/compat_macos/swift-macosx-x86_64/bin/swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename /tmp/test2.swift

1. While type-checking expression at [/tmp/test2.swift:5:27 - line:5:27] RangeText="u"
   Abort trap: 6