I've been trying to squash DIEs that get generated too late, and here 
are some minor fixes.

First, I'm now caching CONST_DECL DIEs.  No sense recreating them in 
late debug.

Second, I've limited gen_generic_params_dies() to early dwarf.  I don't 
think we get any additional location info or anything later.

Last, and similarly as above, imported modules shouldn't need to be 
generated in late debug at all.  I don't think they were, but no sense 
going through them as part of process_scope_var in late debug.

Committed to branch.

Aldy