message: Fix windows build after r369894
  Constructing a std::vector from a llvm::map_range fails on windows,
  apparently because std::vector expects the input iterator to have a
  const operator* (map_range iterator has a non-const one).
  This avoids the cleverness and unrolls the map-loop manually (which is
  also slightly shorter).
diff: @@ -108,12 +108,10 @@ ParseFPOAndStringify(llvm::StringRef prog) {
  llvm::BumpPtrAllocator alloc;
  std::vector<std::pair<llvm::StringRef, Node *>> parsed =
  ParseFPOProgram(prog, alloc);
  - auto range = llvm::map_range(
  - parsed, [](const std::pair<llvm::StringRef, Node *> &pair) {
  - return std::make_pair(pair.first, ASTPrinter::Print(pair.second));
  - });
  - return std::vector<std::pair<std::string, std::string>>(range.begin(),
  - range.end());
  + std::vector<std::pair<std::string, std::string>> result;
  + for (const auto &p : parsed)
  + result.emplace_back(p.first, ASTPrinter::Print(p.second));
  + return result;
  }
  TEST(PostfixExpression, ParseFPOProgram) {
  