[rfc/rfa] accept DW_TAG_namespace and friends, possibly on 5.3

[Daniel Berlin]

> > currently have a version of GCC that emits many
> > DW_TAG_imported_declarations or any DW_TAG_imported_modules; I hope
> > that I'll have one soon (Daniel Berlin is working on it but he's busy;
> > maybe I'll try to work on it myself, too),
> I should have it done in a few hours, actually.

Now I remember why i didn't do it in the first place.
Using decl's aren't common to the tree structure (USING_DECL and 
USING_STMT are in cp/cp-tree.def).

We also never end up walking the statement tree, and there are no debug 
hooks 
for handling each statement anyway (inlined subroutines get handled 
without needing to do this), so it's a loss.
Thus, we'll never see a USING_STMT or USING_DECL.

In other words, this is gonna be complicated and i don't know if it'll be 
accepted, but we'll see.

I can make the DW_TAG_imported_* appear and appear in the right order, but 
not necessarily in the right context.

--Dan