Implement LWG 3013 - some filesystem members should not be noexcept.

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.

llvm-svn: 316941