Use a C++17 feature in QUICHE.

All of our downstream users should support C++17 by now.  If they do, this will work everywhere, and we convert the rest of the codebase to C++17.  If they don't, this will break and the rollback will be limited to this CL.

PiperOrigin-RevId: 421072835

quic/core/http/quic_spdy_stream.cc

diff --git a/quic/core/http/quic_spdy_stream.cc b/quic/core/http/quic_spdy_stream.cc
index 8147ce0..afeeef0 100644
--- a/quic/core/http/quic_spdy_stream.cc
+++ b/quic/core/http/quic_spdy_stream.cc
@@ -1244,9 +1244,7 @@
   std::string protocol;
   absl::optional<QuicDatagramStreamId> flow_id;
   bool version_indicated = false;
-  for (const auto& header : header_list_) {
-    const std::string& header_name = header.first;
-    const std::string& header_value = header.second;
+  for (const auto& [header_name, header_value] : header_list_) {
     if (header_name == ":method") {
       if (!method.empty() || header_value.empty()) {
         return;