[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

utils/release/test-release.sh

@@ -283,7 +283,7 @@ function export_sources() {
         ln -s ../../cfe.src clang
     fi
     cd $BuildDir/llvm.src/tools/clang/tools
-    if [ ! -h clang-tools-extra ]; then
+    if [ ! -h extra ]; then
         ln -s ../../../../clang-tools-extra.src extra
     fi
     cd $BuildDir/llvm.src/projects