Summary:
Update the `XCHECK_EQ()` macros to use `folly::logging::objectToString()` to
format the message on failure.

This ensures that `XCHECK_EQ()` will successfully compile and work with any
arguments.  Previously it would fail to compile if called with arguments that
did not work with `folly::to<std::string>()`

Reviewed By: chadaustin

Differential Revision: D14545301

fbshipit-source-id: 442657a1770f30cbf01462ad62d32301e60490e5