codegen(llvm): emit debug information

Currently, the LLVM code generation backend doesn't implement emitting debug information. This is a must-need feature of any code generation backend and should be implemented.

LLVM provides a way to emit debug information in the DWARF (https://dwarfstd.org/) format. Implementing this will be a useful learning experience for when we implement emitting debug information for other debug formats, and other backends.

Another thing that should be implemented (for debugging information to be useful) is to investigate and implement "unwinding" in the LLVM code generation backend.