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}

rtc_base/thread.cc

diff --git a/rtc_base/thread.cc b/rtc_base/thread.cc
index 692d802..46e082f 100644
--- a/rtc_base/thread.cc
+++ b/rtc_base/thread.cc
@@ -116,7 +116,7 @@
 
 ThreadManager::~ThreadManager() {
   // By above RTC_DEFINE_STATIC_LOCAL.
-  RTC_NOTREACHED() << "ThreadManager should never be destructed.";
+  RTC_DCHECK_NOTREACHED() << "ThreadManager should never be destructed.";
 }
 
 // static