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