This patch adds an implementation of __resize_default_init as
described in P1072R2. Additionally, it uses it in filesystem to
demonstrate its intended utility.
Once P1072 lands, or if it changes it's interface, I will adjust
the internal libc++ implementation to match.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347589 91177308-0d34-0410-b5e6-96231b3b80d8