Enable -Wimplicit-fallthrough (and also fix the discovered issues)

commit_hash:06fd6ffa9ec768168c34547fcdfb15e2f9ad75f8

Author: babenko

library/cpp/yt/ya_cpp.make.inc

@@ -3,5 +3,6 @@
 IF (NOT MSVC)
     CXXFLAGS(
         -Wdeprecated-this-capture
+        -Wimplicit-fallthrough
     )
 ENDIF()

yt/yt/core/concurrency/async_looper.cpp

@@ -187,19 +187,10 @@ void TAsyncLooper::AfterStart(bool cleanStart, bool wasRestarted, ui64 epochNumb
                     // Caller of |Start| will start the new chain
                     // and we just bail out.
                     YT_LOG_DEBUG("Looper restart occured during the intermission between async and sync steps");
-
                     return;
                 }
                 break;
 
-            case EState::Restarting:
-                // Restarting requires stage to
-                // not be |Idle|.
-                // NB(arkady-e1ppa): Since enum is not macro generated
-                // it is not serializable.
-                YT_LOG_ERROR(
-                    "Looper encountered impossible state while starting sync step (State: Restarting)");
-
             default:
                 YT_ABORT();
         }

yt/yt/core/yson/protobuf_interop.cpp

@@ -1151,6 +1151,8 @@ class TProtobufWriter
             switch (field->GetType()) {
                 case FieldDescriptor::TYPE_STRING:
                     ValidateString(value, field->GetFullName());
+                    [[fallthrough]];
+
                 case FieldDescriptor::TYPE_BYTES:
                     BodyCodedStream_.WriteVarint64(value.length());
                     BodyCodedStream_.WriteRaw(value.begin(), static_cast<int>(value.length()));