Use `ln -n` to prevent forming a symlink cycle, instead of rm'ing the source

This is a better fix for the problem fixed in r334972.

Also remove the rm'ing of the symlink destination that was there to
clean up the bots -- it's over a year later, bots should be happy now.

Differential Revision: https://reviews.llvm.org/D64301

llvm-svn: 365414