message: Avoid an llvm::unreachable in the Swift compiler
diff: @@ -1803,6 +1803,13 @@ bool SwiftASTContext::SetTriple(const char *triple_cstr, Module *module) {
  log->Printf("%p: SwiftASTContext::SetTriple('%s') setting to '%s'%s",
  this, triple_cstr, triple.c_str(),
  m_target_wp.lock() ? " (target)" : "");
  +
  + if (llvm::Triple(triple).getOS() == llvm::Triple::UnknownOS) {
  + // This case triggers an llvm_unreachable() in the Swift compiler.
  + if (log)
  + log->Printf("Cannot initialize Swift with an unknown OS");
  + return false;
  + }
  m_compiler_invocation_ap->setTargetTriple(triple);
  // Every time the triple is changed the LangOpts must be
  