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/test/audio_decoder_proxy_factory.h b/test/audio_decoder_proxy_factory.h
index 04f397f6..95606d6 100644
--- a/test/audio_decoder_proxy_factory.h
+++ b/test/audio_decoder_proxy_factory.h
@@ -31,7 +31,7 @@
 
   // Unused by tests.
   std::vector<AudioCodecSpec> GetSupportedDecoders() override {
-    RTC_NOTREACHED();
+    RTC_DCHECK_NOTREACHED();
     return {};
   }