message: [ExpressionParser] Handle llvm::Expected result
  This fixes the unchecked-error assertion at runtime.
  Expected<T> must be checked before access or destruction. Expected<T>
  value was in success state. (Note: Expected<T> values in success mode
  must still be checked prior to being destroyed).
diff: @@ -204,7 +204,11 @@ private:
  llvm::DenseSet<clang::Decl *> m_completed;
  bool ImportAndCheckCompletable(clang::Decl *decl) {
  - (void)m_exporter.Import(decl);
  + llvm::Expected<clang::Decl *> imported_decl = m_exporter.Import(decl);
  + if (!imported_decl) {
  + llvm::consumeError(imported_decl.takeError());
  + return false;
  + }
  if (m_completed.count(decl))
  return false;
  if (!llvm::isa<DeclContext>(decl))
  @@ -225,7 +229,11 @@ private:
  void Complete(clang::Decl *decl) {
  m_completed.insert(decl);
  auto *decl_context = llvm::cast<DeclContext>(decl);
  - (void)m_exporter.Import(decl);
  + llvm::Expected<clang::Decl *> imported_decl = m_exporter.Import(decl);
  + if (!imported_decl) {
  + llvm::consumeError(imported_decl.takeError());
  + return;
  + }
  m_exporter.CompleteDecl(decl);
  for (Decl *child : decl_context->decls())
  if (ImportAndCheckCompletable(child))
  