Summary:
A few fixes:
* missing include: folly/Optional.h
* switch folly::Optional's `has_value()` to `hasValue()` for now until folly is upgraded to newer version
* fix up import for RCTTextAttributes.h
* fix up includes for "conversions.h" to use namespaced includes
Reviewed By: mmmulani
Differential Revision: D8021149
fbshipit-source-id: d3955986d3ab6b1d9b61ac1e385767893ce57e5e
128c9343c4