add FOLLY_MAYBE_UNUSED
add FOLLY_MAYBE_UNUSED

Summary: This adds a macro `FOLLY_MAYBE_UNUSED` in `folly/CppAttributes.h` that can be used to suppress `-Wunused` warnings.

Reviewed By: yfeldblum

Differential Revision: D5444742

fbshipit-source-id: b16e8efefd76282498ad6114bac9548064cf38d5

facebook/folly,Orvid/folly,facebook/folly,Orvid/folly,Orvid/folly,Orvid/folly,rklabs/folly,rklabs/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,Orvid/folly