Properly mark RtpRtcp deprecated

The clang pragma have been added to ensure we can still test the code
until usage is gone, and that we can still have the one implementation
compiling without itself tripping on the deprecation errors.

Users of the code will have deprecation warnings or error as intended.

Bug: webrtc:14617
Change-Id: I21dae57c669557d4d218c235c811174a477be080
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/281221
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38514}
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl.h b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
index 0f4f004..cbe30dd 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
@@ -43,8 +43,12 @@
 struct PacedPacketInfo;
 struct RTPVideoHeader;
 
-// DEPRECATED.
-class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+class ABSL_DEPRECATED("") ModuleRtpRtcpImpl
+    : public RtpRtcp,
+      public RTCPReceiver::ModuleRtpRtcp {
+#pragma clang diagnostic pop
  public:
   explicit ModuleRtpRtcpImpl(
       const RtpRtcpInterface::Configuration& configuration);