replace more folly::Optional::clear by reset (3)
replace more folly::Optional::clear by reset (3)

Summary:
if `foo` must be instantiation of folly::Optional in template functions, this diff replaces

  foo.clear()

by

  foo.reset()

Reviewed By: yfeldblum

Differential Revision: D20208629

fbshipit-source-id: 890cb3df039af20c9428d6b1cf127108538a0b18

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift