Rename and rework `_LIBCPP_TRIVIAL_PAIR_COPY_CTOR`. Move FreeBSD configuration in-tree.

This patch does the following:

* It renames `_LIBCPP_TRIVIAL_PAIR_COPY_CTOR` to `_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR`.
* It automatically enables this option on FreeBSD in ABI V1, since that's the current ABI FreeBSD ships.
* It cleans up the handling of this option in `std::pair`.

I would like the sign off from the FreeBSD maintainers. They will no longer need to keep their `__config` changes downstream.

I'm still hoping to come up with a better way to maintain the ABI without needing these constructors.

Reviewed in https://reviews.llvm.org/D21329

llvm-svn: 275749