Prepare for Subversion migration by implementing a -usesvn to tell the

script to to check out llvm and llvm-test from Subversion instead of CVS.
Without this option the script will continue to check out from CVS. To
specify the Subversion URL, set the SVNURL environment variable or pass
-svnurl followed by the URL. For now, -svnurl will default to Reid's
temporary (read-only, daily snapshot) SVN server. Try it out if you like!

llvm-svn: 35621