this patch breaks some more workers out of pushdecl.

1) in function-scope pushdecl may need to set the decl's context. 
pushdecl had some funky code there to cope with pushing a function decl 
in its own context -- that's no longer needed with my changes to 
start_preparsed_function to get things in the correct order.  But there 
is still some tweaking that needs doing.

2) There are some special rules for function-scope extern decls, they 
can inherit linkage if they have a decl in the enclosing scope.  That 
code was tangled up with that checking for a matching decl, but is 
better separated out, which this patch does too.

3) also introduced a DECL_HIDDEN_P, a useful predicate.

nathan
-- 
Nathan Sidwell