support: Allow generation of llvm-ir, ast and asm according to the compiler flags

[xmnlab]

reference: https://gitlab.com/lfortran/lfortran

• add ast flag
• add llvm-ir flag (ref: https://llvm.org/docs/tutorial/MyFirstLanguageFrontend/LangImpl09.html)
• Create an option for generating an executable file #83
• create the file object (ref: https://llvm.org/docs/tutorial/MyFirstLanguageFrontend/LangImpl08.html)