add opAssign with non-ref param to each IntervalRange
make non-ref opAssigns call the ref overload
change some ddoc comments to /++ Ditto +/
fix fileAttributes for Posix
Merge pull request #1770 from dawgfoto/fixup1751

fix fileAttributes for Posix
Merge pull request #1758 from e10s/issue11681

Fix Issue 11681 - std.datetime.IntervalRange.opAssign with non-ref parameter is required