Replace some bool_constant with constexpr bool in FBVector (#1039)
Replace some bool_constant with constexpr bool in FBVector (#1039)

Summary:
- Some constant expressions are wrapped in a type carrying a bool, but
  are only used in value contexts. Remove the type wrapper
  (bool_constant) use and just use `constexpr bool` directly. This makes the
  usages not have to use `::value` everywhere.
Pull Request resolved: https://github.com/facebook/folly/pull/1039

Reviewed By: Orvid

Differential Revision: D14293828

Pulled By: yfeldblum

fbshipit-source-id: 78399e4fb401ab680a4f1ecef63fb4c2a82927c2

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