Bug#1068961: llvm-toolchain-18: block migration to testing: to many llvm-toolchain versions in testing

[Drew Parsons]

Source: llvm-toolchain-18
Followup-For: Bug #1068961

To give some more context to the issue of llvm-toolchain versions,
we need to keep llvm-toolchain-14 in the archive (for the time being)
since it is required by llvmlite.  llvmlite is used by numba, which
would offer valuable acceleration to python numerical packages,
if the numba/llvmlite/llvm situation were more reliable.

The reasons why llvmlite is stuck on the old version of llvm
are discussed at
https://llvmlite.readthedocs.io/en/latest/faqs.html#why-doesn-t-llvmlite-always-support-the-latest-release-s-of-llvm
https://github.com/numba/llvmlite/issues/1048
https://github.com/numba/llvmlite/issues/1062
see also https://github.com/numba/llvmlite/issues/914

In short, llvm is unstable (in the sense of changing API every
release), and llvmlite does not have the resources to keep up with the
ongoing changes.

If possible, it would be sensible in principle for debian to drop
llvm-toolchain-15 and llvm-toolchain-16.
i.e. only keep llvm-toolchain-14 (for llvmlite/numba)
and the latest llvm-toolchain
(likewise drop llvm-toolchain-17 once llvm-toolchain-18 is available in testing).

Perhaps other packages have the same issue as llvmlite, that they need
specific older versions of llvm.