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/strings/string_builder.cc

@@ -98,7 +98,7 @@ SimpleStringBuilder& SimpleStringBuilder::AppendFormat(const char* fmt, ...) {
   } else {
     // This should never happen, but we're paranoid, so re-write the
     // terminator in case vsnprintf() overwrote it.
-    RTC_NOTREACHED();
+    RTC_DCHECK_NOTREACHED();
     buffer_[size_] = '\0';
   }
   va_end(args);