Add tips for working with LLVM.

This PR adds Julia-specific advice about LLVM to  `devdocs`, culled from my notes from working on Julia.  Hopefully others can extend it in the future.
i wonder if we should just list all of the source files and a brief description. otherwise `gf == generic function` is rather non-obvious. for cpp files, there's also:
`debuginfo.cpp` Tracks debug information for JIT code
`disasm.cpp` Handles native object file and JIT code disassembly (aka `code_native`)