Currently the `lfortran.codegen.gen` uses the old AST+annotations approach. We need to extend `lfortran.codegen.asr_to_llvm` which converts the ASR to LLVM to be able to handle all the constructs that `lfortran.codegen.gen` can, and then remove `lfortran.codegen.gen`. As part of this, once `lfortran.codegen.gen` is removed, also remove `lfortran.semantic.analyze` and only keep `lfortran.semantic.ast_to_asr`.