Introduce RTC_CHECK_NOTREACHED(), an always-checking RTC_NOTREACHED()

And use it in a few places that were using RTC_CHECK(false) or FATAL()
to do the exact same job. There should be no change in behavior.

Bug: none
Change-Id: I36d5e6bcf35fd41534e08a8c879fa0811b4f1967
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/191963
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32567}

modules/audio_processing/test/test_utils.cc

@@ -146,8 +146,7 @@ AudioProcessing::ChannelLayout LayoutFromChannels(size_t num_channels) {
     case 2:
       return AudioProcessing::kStereo;
     default:
-      RTC_CHECK(false);
-      return AudioProcessing::kMono;
+      RTC_CHECK_NOTREACHED();
   }
 }