Summary:
- Change folly::coro::co_viaIfAsync to be a customization-point object.
- Modify co_viaIfAsync() to call a .viaIfAsync() member function if one exists.
  Otherwise it calls the co_viaIfAsync() function found by ADL.
- Add FOLLY_DEFINE_CPO() helper macro to folly/Portability.h
  Borrows some techniques from range-v3 to support CPOs in different
  compilers and C++ versions.
- Redefined co_viaIfAsync() CPO in terms of this macro.
  This allows it to be callable as folly::coro::co_viaIfAsync() while
  still allowing other types in folly::coro namespace to define friend
  functions that customise the behaviour of this customisation point
  without causing conflicts.
- Add folly::coro::is_semi_awaitable<T> metafunction.
- Add folly::coro::semi_await_result_t<T> metafunction.

Reviewed By: yfeldblum

Differential Revision: D14016389

fbshipit-source-id: 63998993b2fa8560700f81666a910dee31f3ba72