Fix "C4715 not all control paths return a value" warning in MSVC (#45827)

Summary:
When integrating react-native into react-native-windows, we got the following build warning (which we treat as an error) when building ReactCommon: `C4715 not all control paths return a value`

This PR adds defaults to the switches to make sure every path returns a value.

See microsoft/react-native-windows#13516

## Changelog:

[GENERAL] [FIXED] Fix "C4715 not all control paths return a value" warning in MSVC when building ReactCommon

Pull Request resolved: #45827

Test Plan: The switches are checking enums this code should never be hit unless new enum values are added.

Reviewed By: robhogan

Differential Revision: D61103286

Pulled By: NickGerleman

fbshipit-source-id: 2028cb60e0b438b9ac17a828f5e1b690052a0bec

Author: jonthysell

packages/react-native/ReactCommon/react/renderer/core/EventQueueProcessor.cpp

@@ -59,6 +59,7 @@ void EventQueueProcessor::flushEvents(
             return hasContinuousEventStarted_ ? ReactEventPriority::Continuous
                                               : ReactEventPriority::Default;
         }
+        return ReactEventPriority::Default;
       }();
     } else {
       if (event.category == RawEvent::Category::ContinuousEnd) {

packages/react-native/ReactCommon/react/runtime/TimerManager.cpp

@@ -48,6 +48,7 @@ inline const char* getTimerSourceName(TimerSource source) {
     case TimerSource::RequestAnimationFrame:
       return "requestAnimationFrame";
   }
+  return "unknown";
 }
 
 } // namespace