Expose accidentally removed __compressed_pair constructor taking piecewise_construct_t.  This fixes http://llvm.org/bugs/show_bug.cgi?id=15918 .

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181217 91177308-0d34-0410-b5e6-96231b3b80d8