Standardize use of filesystem

If <filesystem> exists, use that, if it doesn't, try
<experimental/filesystem>. Either way, access it from std via a
namespace splice.

Change-Id: I5b35ee9f74b88e188cb3ca3fed9abaec64933697
Signed-off-by: Vernon Mauery <vernon.mauery@intel.com>
6 files changed