Move xplat/sonar/xplat/Sonar* to Flipper*

Summary:
Part of the Sonar -> Flipper rename
Intentionally left externally visible buck target as Sonar for now to minimize diff size.

Reviewed By: passy

Differential Revision: D9871684

fbshipit-source-id: 6926eb62c578a05cd895745c75ed7da3cfe965b5

xplat/Flipper/Log.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <string>
+
+namespace facebook {
+namespace flipper {
+
+  void log(const std::string& message);
+
+} // namespace flipper
+} // namespace facebook