[libcxx] Fix __compressed_pair so it doesn't copy the argument multiple times, and add constexpr.

Summary:
__compressed_pair takes and passes it's constructor arguments by value. This causes arguments to be moved 3 times instead of once. This patch addresses that issue and fixes `constexpr` on the constructors.

I would rather have this fix than D27564, and I'm fairly confident it's not ABI breaking but I'm not 100% sure.

I prefer this solution because it removes a lot of code and makes the implementation *much* smaller.

Reviewers: mclow.lists, K-ballo

Reviewed By: K-ballo

Subscribers: K-ballo, cfe-commits

Differential Revision: https://reviews.llvm.org/D27565

llvm-svn: 300140