avoid optionality underlying-type in cpp2 reflection serializer
avoid optionality underlying-type in cpp2 reflection serializer

Summary: There is no purpose to using the underlying-type of the optionality enum in the way that is done, since templates may be parameterized, specialized, and instantiated with enumerations.

Reviewed By: vitaut

Differential Revision: D26479487

fbshipit-source-id: 5abe0cfdfb9194a1119d20d270bebb3cb03a0e76

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