LWG 3013 points out that the constructors and increment members
of the directory iterators need to allocate, and therefore cannot
be marked noexcept.
It also points out that `is_empty` and `copy` likely need to allocate
as well, and as such can also not be noexcept.
This patch speculatively implements the resolution removing noexcept,
because libc++ does indeed have the possibility of throwing on allocation
failure.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@316941 91177308-0d34-0410-b5e6-96231b3b80d8
a4c272d82f