Summary:
Older standard libraries provide a `std::enable_shared_from_this` that doesn't have a `weak_from_this()` member, even when compiling in C++17 mode. Change the preprocessor conditional to account for this, and select the custom Folly implementation when the `std::` one is non-conforming.

This change makes special accommodation for older MSVC versions, which fails to define the `__cpp_lib_enable_shared_from_this` feature test macro, but that provides a conforming `std::enable_shared_from_this` when compiling as C++17.

Reviewed By: yfeldblum, ispeters

Differential Revision: D25402425

fbshipit-source-id: c53b9fcebbf93c54342824dd56fd11eaa681abd1