LLVM > 15

We should upgrade PhASAR's required maximum LLVM version to something >15 to profit from API improvements and for compatibility with other tools, e.g., the IR used in `rustc`.

We should start with upgrading LLVM to 16.


### Prerequisites:
- With LLVM 16, our clone of LLVM's CFLAndersAnalysis does not work anymore. We should either fix or replace it,