[patch] Fix symtab/14601

[Tom Tromey]

Doug> Not all code paths to handle DW_TAG_imported_{declaration,module}
Doug> ensure using_directives is reset to NULL.
Doug> This patch fixes things by "going with the flow" and resetting
Doug> using_directives to NULL in buildsym_init along with the others.

What code paths result in the bad behavior?
I think this patch is fine but I wonder if the underlying bug has other
possible bad effects.

Tom