Include file information in emitted debug info

I was debugging https://github.com/dcjones/Gadfly.jl/issues/206 and was briefly misled by a wrong backtrace for macros in the debugger because we don't emit the file info. This fixes that. Might also help https://github.com/JuliaLang/julia/issues/1334 (@timholy because of that). 
Should this default to `SP` to inherit the filename from the top of the function?