[Release Script] Check for correct symlink name

[Release Script] Check for correct symlink name

While checking for the existence of the clang-tools-extra directory,
the script was not checking for its destination name, "extra", and
the script was failing when re-running without checking out new
sources.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243898 91177308-0d34-0410-b5e6-96231b3b80d8