Update LLVM add-apt-repository patch

[aminya]

The downloaded llvm.sh script contains add-apt-repository -y "${REPO_NAME}" so I don't think this replacement will be done.
The script also has an update afterwards which could be patch instead of adding the apt-get update.

See:
https://github.com/opencollab/llvm-jenkins.debian.net/blob/master/llvm.sh#L173-L174

Originally posted by @bjosv in #257 (comment)

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