Rename qpack_constants.* to qpack_instructions.*.

There used to be some constants in this file not related to QPACK instructions,
but removing them has left these two files inaccurately named.

gfe-relnote: n/a, rename only.
PiperOrigin-RevId: 282819395
Change-Id: If7a6868836ec6d02533f662a42a30e274e68b29d
diff --git a/quic/core/qpack/qpack_instruction_decoder_test.cc b/quic/core/qpack/qpack_instruction_decoder_test.cc
index 54fa416..c066827 100644
--- a/quic/core/qpack/qpack_instruction_decoder_test.cc
+++ b/quic/core/qpack/qpack_instruction_decoder_test.cc
@@ -6,7 +6,7 @@
 
 #include <algorithm>
 
-#include "net/third_party/quiche/src/quic/core/qpack/qpack_constants.h"
+#include "net/third_party/quiche/src/quic/core/qpack/qpack_instructions.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_logging.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_test.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_text_utils.h"