Add debugging information

Apparently, MCJIT would allow us to expose debug information so that JITted functions aren't seen as opaque under supported debuggers (such as gdb).
Unfortunately, since we aren't using the C++ API for building module code, it seems we'll have to manually emit metadata in some complicated format. But at least it should be possible.

Reference:
- http://llvm.org/docs/DebuggingJITedCode.html
- http://llvm.org/docs/SourceLevelDebugging.html
- http://llvm.org/docs/doxygen/html/classllvm_1_1DIBuilder.html

Start with the linked LLVM documentation on debugging JITed code, source-level debugging, and DIBuilder, then inspect llvmlite’s Python-side module-building and MCJIT integration. Done means JITted functions expose usable debug information instead of appearing opaque under supported debuggers such as gdb.

Written by the indexing model from the issue text.