Support LLVM 20

[DarkTyger]

Please add support for clang 20 (or greater):

cd /tmp
git clone https://github.com/NASA-SW-VnV/ikos
cd ikos
md build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/tmp/ikos/dist ..
make

Output:

Found Boost: /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0 (found version "1.55.0")
Found Boost components:
   filesystem;system

-- Found LLVM: /usr/lib/llvm-20 (found version "20.0.0")
CMake Error at frontend/llvm/CMakeLists.txt:107 (message):
  LLVM 14 is required.

[ivanperez-keera]

We are trying to work our way up in LLVM versions from the current one (version 14).

Is this something you could help with?