TarWriter: Use Ustar header's "prefix" field to store long filenames.

Tar's Ustar header has the "prefix" field to store a directory
part of a filename. It is not as flexible as the PAX-extended
filename because there's still a limitation on the maximum filename
size, but it mitigates the situation.

This patch should unbreak some Windows buildbots that uses very
old tar command.

llvm-svn: 291340