[cfg-branch]: Outputs dwarf2 location lists using variable tracking info

[Richard Henderson]

> Should this be handled in the debug writer, just 
> replacing location/merging range until we get to a different source line
> source line ? Or should the variable tracking pass make it's own line 
> easier/not generate as many notes by only considering changes that 
> occur once we see a line note?

Probably the debug writer.  Also, I think it might make sense
to allow the full insn-by-insn tracking at -g3 (or something).


r~