Removing duplicate DWARF2 info

[Daniel Berlin]

> 
> Do you want to teach every makefile in existence about this persistent
> hash table?  Which will probably require adding features to make, and
> therefore locking every makefile in existence into one implementation
> of make - which is a thing we try ridiculously hard not to do?
> 
> And don't think you can get away with not telling make about it,
> either.  Consider what happens when you clean out a build directory -
> and that's the *simplest* case.
> 

When you clean out the build directory, you lose the info.
You lost the objects too, so what's the problem.
It's like -frepo, but for debug info instead.
> zw
>