Add another minor hack to cooked_index_entry::full_name

This patch adds another minor hack to cooked_index_entry::full_name.
In particular, if GNAT emits non-hierarchical names (still the default
as the hierarchical series is blocked on one tricky problem), then a
request to compute the "linkage-style" name will now just return the
'name' field.

Without this tweak, this series would regress ada-cold-name.exp,
because the search would look for "name.cold" but the index would
return "name[cold]" as the "linkage" name (which would be wrong).

This area is a bit difficult to unravel.  The best plan here, IMO, is
to change Ada to work like the other languages in gdb: store the
natural name and do searches with that name.  I think this is
achievable, but I didn't want to try it here.

I've updated the relevant bug (tagged below) to reflect this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32766
Acked-By: Simon Marchi <simon.marchi@efficios.com>