Add folly non null

Summary:
Add non null annotation to
folly to help with linters
complaining when a pointer could
be null, however it never is.

Reviewed By: yfeldblum

Differential Revision: D5453255

fbshipit-source-id: cf7ccbcc86d171ef4f6d373b6f751f709e4587b1

folly/CppAttributes.h

@@ -90,6 +90,8 @@
  */
 #if FOLLY_HAS_EXTENSION(nullability)
 #define FOLLY_NULLABLE _Nullable
+#define FOLLY_NONNULL _Nonnull
 #else
 #define FOLLY_NULLABLE
+#define FOLLY_NONNULL
 #endif