is duplicated by:

• SR-12017 Tab completion in repl causes "Segmentation fault: 11"

Issue Description:

If I try tab completion in the REPL in this example:

$ swift
Welcome to Apple Swift version 5.1 (swiftlang-1100.0.270.13 clang-1100.0.33.7).
1> var optionalName: String? = "Foo"
optionalName: String? = "Foo"
2> if let name = optio<tab>

I get the following seg fault error:

lldb 0x000000010b8c2705 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1 lldb 0x000000010b8c1d77 llvm::sys::RunSignalHandlers() + 39
2 lldb 0x000000010b8c2d58 SignalHandler(int) + 264
3 libsystem_platform.dylib 0x00007fff78587b5d _sigtramp + 29
4 libsystem_platform.dylib 0x00007fdb6d27c300 _sigtramp + 4107225024
5 LLDB 0x000000010c37b1dd (anonymous namespace)::CodeCompletionCallbacksImpl::doneParsing() + 15469
6 LLDB 0x000000010cb597ba swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 458
7 LLDB 0x000000010c39f03e doCodeCompletion(swift::SourceFile&, llvm::StringRef, unsigned int*, swift::CodeCompletionCallbacksFactory*) + 494
8 LLDB 0x000000010c39ede1 swift::REPLCompletions::populate(swift::SourceFile&, llvm::StringRef) + 673
9 LLDB 0x000000010bdde029 lldb_private::SwiftREPL::CompleteCode(std::_1::basic_string<char, std::1::char_traits<char>, std::_1::allocator<char> > const&, lldb_private::StringList&) + 569
10 LLDB 0x000000010bb8d4a9 lldb_private::REPL::IOHandlerComplete(lldb_private::IOHandler&, char const*, char const*, char const*, int, int, lldb_private::StringList&, lldb_private::StringList&) + 489
11 LLDB 0x000000010bb01157 lldb_private::IOHandlerEditline::AutoCompleteCallback(char const*, char const*, char const*, int, int, lldb_private::StringList&, lldb_private::StringList&, void*) + 55
12 LLDB 0x000000010bba41fe lldb_private::Editline::TabCommand(int) + 110
13 LLDB 0x000000010bba6531 lldb_private::Editline::ConfigureEditor(bool)::$13::_invoke(editline*, int) + 33
14 libedit.3.dylib 0x00007fff75f0774b el_wgets + 478
15 libedit.3.dylib 0x00007fff75f1294b el_gets + 37
16 LLDB 0x000000010bba5d1e lldb_private::Editline::GetLines(int, lldb_private::StringList&, bool&) + 446
17 LLDB 0x000000010bb01b03 lldb_private::IOHandlerEditline::Run() + 115
18 LLDB 0x000000010bae8fdb lldb_private::Debugger::ExecuteIOHandlers() + 123
19 LLDB 0x000000010baeba23 lldb_private::Debugger::IOHandlerThread(void*) + 19
20 LLDB 0x000000010bb962d7 lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) + 103
21 libsystem_pthread.dylib 0x00007fff785902eb _pthread_body + 126
22 libsystem_pthread.dylib 0x00007fff78593249 _pthread_start + 66
23 libsystem_pthread.dylib 0x00007fff7858f40d thread_start + 13
Segmentation fault: 11