Integrate improvements from NMODL `llvm` branch to `master`

[alexsavulescu]

Currently there are differences between the llvm branch of NMODL (https://github.com/BlueBrain/nmodl/tree/llvm) and master in multiple places (i.e. AST classes, Codegen visitors, etc). We need to check these differences, identify which are improvements for NMODL and try to reduce the diff and the technical debt for the time llvm will get merge to master. Also note that there are differences between the LLVM 13.0.x API that llvm branch is based and more recent LLVM versions that need to be handled when merging the two branches