Uses a swap operation to completely clear `std::deque` state.

The `std::deque` shows up in prod heap profiles, even though it's likely empty after the connection goes forward-secure.

Protected by data structure type change, not possible to protect.

PiperOrigin-RevId: 845929151

quiche/quic/core/quic_connection.cc

diff --git a/quiche/quic/core/quic_connection.cc b/quiche/quic/core/quic_connection.cc
index 1ff58d4..a1148dc 100644
--- a/quiche/quic/core/quic_connection.cc
+++ b/quiche/quic/core/quic_connection.cc
@@ -4615,7 +4615,7 @@
             undecryptable_packet.encryption_level, /*dropped=*/true);
       }
     }
-    undecryptable_packets_.clear();
+    std::deque<UndecryptablePacket>().swap(undecryptable_packets_);
   }
   if (perspective_ == Perspective::IS_CLIENT) {
     SetRetransmissionAlarm();