Prepare to rename RTC_NOTREACHED to RTC_DCHECK_NOTREACHED

Add implementation of RTC_DCHECK_NOTREACHED equal to the RTC_NOTREACHED.
The new macros will replace the old one when old one's usage will be
removed. The idea of the renaming to provide a clear signal that this
is debug build only macros and will be stripped in the production build.

Bug: webrtc:9065
Change-Id: I4c35d8b03e74a4b3fd1ae75dba2f9c05643101db
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/237802
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35348}
diff --git a/video/video_stream_encoder_unittest.cc b/video/video_stream_encoder_unittest.cc
index 49e5619..1331b8c 100644
--- a/video/video_stream_encoder_unittest.cc
+++ b/video/video_stream_encoder_unittest.cc
@@ -8382,7 +8382,7 @@
             false);
         break;
       default:
-        RTC_NOTREACHED();
+        RTC_DCHECK_NOTREACHED();
     }
     ConfigureEncoderAndBitrate(codec_type_, std::move(encoder));
   }
@@ -8609,7 +8609,7 @@
       str = "Multiplex";
       break;
     default:
-      RTC_NOTREACHED();
+      RTC_DCHECK_NOTREACHED();
   }
   str += allow_i420_conversion ? "_AllowToI420" : "_DisallowToI420";
   return str;