Use maximum compression when packaging release tarballs.

Summary:
Since a full run of test-release.sh takes many hours (at least on my
poor systems), we might as well spend some extra time compressing the
tarball, in return for a quite a bit of gains for uploading and
downloading it.

As an example, the 10.0.0-rc4 .tar.xz tarball shrinks from 465MiB to
306MiB, about 52% smaller.

Reviewers: hans, tstellar, rovka

Reviewed By: hans

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76192

llvm/utils/release/test-release.sh

@@ -470,9 +470,9 @@ function package_release() {
     cd $BuildDir/Phase3/Release
     mv llvmCore-$Release-$RC.install/usr/local $Package
     if [ "$use_gzip" = "yes" ]; then
-      tar cfz $BuildDir/$Package.tar.gz $Package
+      tar cf - $Package | gzip -9c > $BuildDir/$Package.tar.gz
     else
-      tar cfJ $BuildDir/$Package.tar.xz $Package
+      tar cf - $Package | xz -9ce > $BuildDir/$Package.tar.xz
     fi
     mv $Package llvmCore-$Release-$RC.install/usr/local
     cd $cwd