Summary:
Renaming FBMacros.h to FBDefines.h for more standard naming (similar to say UIKit/UIKit.h or Foundation/Foundation.h).
```
Tools/fastmod/fastmod --accept-all '\bFBMacros.h' FBDefines.h ../xplat
```
drop-conflicts
bypass-lint
allow-large-files
Reviewed By: adamjernst
Differential Revision: D14011897
fbshipit-source-id: f1dd7e686eef2cdb109e85c2db4a0f2ac3d0ba42