How to use the new clang packages

[Rafael Ávila de Espíndola]

For some time now we have had experimental clang packages in
/tools/clang-* in the bots. They were convenient to use on try, but
extremely expensive to upgrade.

We have now switched to having manifest files in m-c itself that list
the cheksum of some files and the bots download the files in those
manifests automatically. This reduces the upgrade time a lot and might
improve a bit more in the future.

In this new process one has to modify the
browser/config/tooltool-manifests/*/releng.manifest manifests in the try
push. To make this easy, we are checking in clang manifests in
browser/config/tooltool-manifests/*/clang.manifest. Just copy over the
ones you need.

With the manifests the clang package is no longer in /tools, it instead
shows up in $topsrcdir/clang, so you can select it with

+export CC=$topsrcdir/clang/bin/clang
+export CXX=$topsrcdir/clang/bin/clang++

I have attached a patch that was pushed to try to test this.

Thanks a lot to Rail for making this possible.

Cheers,
Rafael