Pull in r242377 from upstream libc++ trunk (by Marshall Clow):

Pull in r242377 from upstream libc++ trunk (by Marshall Clow):

  Make sure that __libcpp_compressed_pair_imp default-constructs its'
  members, rather than value-initializing them. Fixes PR#24137

This ensures std::make_shared<>'s default constructor properly
initializes to zero.

Direct commit to stable/9 and stable/10, since stable/11 and head
already have a newer version of libc++, including this fix.

Reported by:	martin.beran@kernun.cz
PR:		217200

git-svn-id: svn://svn.freebsd.org/base/stable/9@313916 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f