Lowering debug symbols to LLVM

[resetius]

Currently, Qumir does not emit debug symbols in LLVM IR. For source-level debugging, we need to propagate source locations (line/column info) through all compiler layers down to LLVM.

Design Ideas

• AST Annotation: Attach source location info (line, column, file) to all AST nodes during parsing (done)
• IR Propagation: Preserve location info in IR nodes during lowering and transformations.
• LLVM Metadata: At codegen, emit LLVM !dbg metadata for instructions, variables, and functions using DWARF standard.
• Mapping Table: Maintain mapping from Qumir source lines to LLVM instructions for error reporting and debugging.
• Pragmas/Flags: Allow enabling/disabling debug info via pragma or compiler flag.