Extract LLVM build

[BBBSnowball]

This should be a seperate build step that results in an artifact that we can put into Artifactory. It should only be rebuild, if we change LLVM.

These are the subtasks:

1. Build LLVM and install it to a temporary location
2. Make at least one artifact of the installed files. It should contain everything we need to run LLVM or compile passes. Ideally, we have one artifact per concern, e.g. llvm and llvm-dev.
3. Extract our passes and put them into a location outside of the LLVM tree. Build them without building all of LLVM. Ideally, they should only use the installed libraries and headers, i.e. they don't need the LLVM sources.
4. Put the artifacts into Artifactory and retrieve and unpack them, when we need them.
5. Only rebuild LLVM, if we have to. This means that we either check for an up-to-date artifact before we build or we extract the build step in Jenkins and create a pipeline.

@ssurmund: Please assign the issue to me, when (1) to (3) are done. I can do (4) and (5).