Fix index's handling of DW_TAG_imported_declaration

Currently the full symbol reader puts DW_TAG_imported_declaration in
TYPE_DOMAIN, in the global scope.  This patch changes the cooked
indexer to follow.

Without this patch, a later patch in the series would cause
nsalias.exp to regress.

This also updates read-gdb-index.c to do something similar.

Acked-By: Simon Marchi <simon.marchi@efficios.com>