Fix compilation issues on Apple platforms. (#1641)
Fix compilation issues on Apple platforms. (#1641)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/1641

There's a bug in C++ SDK shipped on Apple platforms that marks `std::optional`'s `value()` as unavailable. To resolve that we should use `operator*` instead.

Reviewed By: tcw165

Differential Revision: D30755147

fbshipit-source-id: 4edcf60a7d29abe7eaddceb66501535031553678

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly