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}

rtc_base/numerics/safe_conversions.h

@@ -63,12 +63,10 @@ inline constexpr Dst saturated_cast(Src value) {
 
     // Should fail only on attempting to assign NaN to a saturated integer.
     case internal::TYPE_INVALID:
-      FATAL();
-      return std::numeric_limits<Dst>::max();
+      RTC_CHECK_NOTREACHED();
   }
 
-  FATAL();
-  return static_cast<Dst>(value);
+  RTC_CHECK_NOTREACHED();
 }
 
 }  // namespace rtc