Don't check that std::pair is trivially copyable on FreeBSD

As FreeBSD already used libc++ before it changed its ABI, we still use
the non-trivially copyable version of std::pair, which used to be
exposed via `_LIBCPP_TRIVIAL_PAIR_COPY_CTOR`, but more recently via
`_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR`.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D99834