Add language to cooked_index_entry

This adds a new 'lang' member to cooked_index_entry.  This holds the
language of the symbol.  This is primarily useful for the new
.debug_names reader, which will not scan the CUs for languages up
front.

This also changes cooked_index_shard::add to return a non-const
pointer.  This doesn't impact the current code, but is needed for the
new reader.