Fix std::pair on FreeBSD

Summary:
FreeBSD ships an old ABI for std::pair which requires that it have
non-trivial copy/move constructors. Currently the non-trivial
copy/move is achieved by providing explicit definitions of the
constructors. This is problematic because it means the constructors
don't SFINAE properly. In order to SFINAE copy/move constructors they
have to be explicitly defaulted and hense non-trivial.

This patch attempts to provide SFINAE'ing copy/move constructors for
std::pair while still making them non-trivial. It does this by adding
a base class with a non-trivial copy constructor and then allowing
pair's constructors to be generated by the compiler. This also allows
the constructors to be constexpr.

Reviewers: emaste, theraven, rsmith, dim

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25389