Support: Reduce stats in fs::copy_file on Darwin

fs::copy_file() on Darwin has a nice optimization to clone the file when
possible. Change the implementation to use clonefile() directly, instead
of the higher-level copyfile().  The latter does the wrong thing for
symlinks, which requires calling `stat` first...

With that out of the way, optimistically call clonefile() all the time,
and then for any error that's recoverable try again with copyfile()
(without the COPYFILE_CLONE flag, as before).

Differential Revision: https://reviews.llvm.org/D112250