Correctly restore whether or not a function is an inner generator

Summary:
If a generator was large enough to be lazily compiled, we would lose
that information when reconstituting the function's context. This meant
the function was generated as a regular function instead of a generator.

#utd-hermes-ignore-android

Reviewed By: tmikov

Differential Revision: D23580247

fbshipit-source-id: af5628bf322cbdc7c7cdfbb5f8d0756328518ea1