apt: add clang + llvm-dev to the full image for OpenModelica LLVM builds

[SVAGEN26]

Summary

Adds clang, llvm, and llvm-dev to COMMON_PKGS in apt/Dockerfile so the published build-deps:*-full images ship the LLVM toolchain out of the box.

Motivation

The OpenModelica revive-llvm-jit PR (#15890) and downstream LLVM work want -DOM_OMC_ENABLE_LLVM=ON to be reachable from CI without further provisioning. Today the full image has neither clang nor llvm-dev, so CMake's find_package(LLVM 18 CONFIG …) fails on the base image and any LLVM-enabled build has to install these itself.

What CMake actually needs

OMCompiler/Compiler/runtime/CMakeLists.txt:147-171 requires:

• LLVM ≥ 16 (asserted at line 154)
• LLVMConfig.cmake (provided by llvm-N-dev)
• Components: core orcjit mcjit executionengine transformutils ipo support irreader bitwriter native nativecodegen
• clang + llvm-link reachable via LLVM_TOOLS_BINARY_DIR (used by EXT_LLVM.compileModelToBitcode at simulate time to lower model C to bitcode)

The llvm, llvm-dev, clang meta-packages pull in the distro-default LLVM major with all of the above.

Distro coverage

The meta-packages resolve to whatever ships natively; no third-party apt repo is added. Consequences per image in .ci/matrix.yml:

image	LLVM major	OM_OMC_ENABLE_LLVM=ON usable?
ubuntu 26.04	20	yes
ubuntu 24.04	18	yes
ubuntu 22.04	14	no (CMake refuses at line 154)
debian 13	19	yes
debian 12	14	no (CMake refuses at line 154)

The default OpenModelica build (LLVM off) is unaffected. On the two older distros, an LLVM-on build fails with a clear "needs LLVM >= 16" message rather than being silently degraded, which is the right behavior. If a follow-up wants LLVM ≥ 16 on Jammy or Bookworm, adding apt.llvm.org in a separate conditional RUN is the natural next step; that is out of scope for this PR.

Image size impact

llvm-N-dev and the LLVM shared libs are large (~500 MB on Ubuntu 24.04). The full image already carries Qt6, TeX Live, and the OpenCL stack; the delta is meaningful but not disproportionate for a base image whose purpose is "everything needed to build OpenModelica."

Test plan

• docker build --target full --build-arg DISTRO=ubuntu --build-arg VERSION=24.04 --tag build-deps:ubuntu-24.04 apt succeeds.
• Inside that image, dpkg -l | grep -E 'llvm-|clang' shows llvm-18-dev and clang-18 (or the distro-default equivalent).
• apt list --installed 2>/dev/null | grep -q llvm-18-dev.
• Inside that image, cloning OpenModelica revive-llvm-jit and configuring with -DOM_OMC_ENABLE_LLVM=ON -DOM_OMC_ENABLE_LLVM_JIT=ON finds LLVM ≥ 16.
• docker build --target full --build-arg DISTRO=ubuntu --build-arg VERSION=22.04 --tag build-deps:ubuntu-22.04 apt still succeeds (older LLVM installed; no build regression on the default image, only OM_OMC_ENABLE_LLVM=ON would refuse there).

Refs OpenModelica/OpenModelica#15890

[AnHeuermann]

clang is already part of build-deps from OpenModelica.

Do we need llvm and llvm-dev for all builds of OpenModelica or should it be an add-on like debug in https://github.com/OpenModelica/build-deps/blob/main/apt/Dockerfile#L270-L287?

Otherwise looks fine. We can merge it as soon as the build and publish step is fixed in our CI.