Fix "control reaches end of non-void function" warnings

"warning: control reaches end of non-void function [-Wreturn-type]"
Reported by gcc (8.3)

In all the reported cases, the end of function is never actually
reached. Add RTC_CHECK(false) to ensure the compiler is aware that
this path is a dead-end.

Bug: webrtc:12008
Change-Id: I7f816fde3d1897ed2774057c7e05da66e1895e60
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/189784
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Fabien VALLÉE <fabien.vallee@netgem.com>
Cr-Commit-Position: refs/heads/master@{#32503}
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index 849a0feb..33f2188 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -114,6 +114,7 @@
     case Agc1Config::kFixedDigital:
       return GainControl::kFixedDigital;
   }
+  RTC_CHECK(false);
 }
 
 // Maximum lengths that frame of samples being passed from the render side to
@@ -1825,9 +1826,8 @@
               return NsConfig::SuppressionLevel::k18dB;
             case NoiseSuppresionConfig::kVeryHigh:
               return NsConfig::SuppressionLevel::k21dB;
-            default:
-              RTC_NOTREACHED();
           }
+          RTC_CHECK(false);
         };
 
     NsConfig cfg;