Pin LLVM / clang version to TDB (#613)
Pin LLVM / clang version to TDB (#613)

The current LLVM / clang install procedure always fetches the latest LLVM release when the build Docker container is built.
This means we will get random upgrades of the toolchain which will introduce incompatible changes (e.g. clang-format rules change) and will fail our CI builds spontaneously and without clear indication.

This PR pins the LLVM version to a known working value (TBD), so we can do planed upgrades at our pace.
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