Switch to unified Truffle+Graal repo

[smarr]

This change solves part of the issues discussed in #133.
It updates libs/truffle to the latest version, which is the unified Graal+Truffle repository.

Note, when updating after this change is merged, try to do the following before updating (or corresponding actions in other tools):

# remove ide related files
cd libs/truffle
../mx/mx ideclean
cd ../..

# now let's try to update
git pull
git submodule update
ant ideinit

Note, submodules are now configured to do shallow checkouts.
I hope this makes a first clone faster.
Also, the Eclipse project now uses the Truffle project to find the annotation processor jars, which hopefully makes it a bit more robust for other folder changes.

This PR does not yet include any changes to use the Graal version included in the repo for benchmarks.

@ctrlpz @daumayr @Richard-Roberts once this is merged, please note that updating might result in errors. So, please see the note above.

[coveralls]

Coverage decreased (-0.004%) to 79.79% when pulling 26bf0d3 on use-unified-graal-repo into 46929f1 on master.