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_coding/codecs/ilbc/audio_encoder_ilbc.cc

@@ -34,7 +34,7 @@ int GetIlbcBitrate(int ptime) {
       // 50 bytes per frame of 30 ms => (approx) 13333 bits/s.
       return 13333;
     default:
-      FATAL();
+      RTC_CHECK_NOTREACHED();
   }
 }
 
@@ -144,7 +144,7 @@ size_t AudioEncoderIlbcImpl::RequiredOutputSizeBytes() const {
     case 6:
       return 2 * 50;
     default:
-      FATAL();
+      RTC_CHECK_NOTREACHED();
   }
 }