[PATCH] don't use chainon when building VMS descriptors in Ada FE

[Eric Botcazou]

> This patch gives the Ada FE the same treatment the Fortran and ObjC
> frontends have recently undergone: removing chainon when building fields
> lists for TYPE_FIELDS.  This change eliminates quadratic behavior and
> makes a future refactoring of TREE_CHAIN easier.

The usage of chainon is indeed bogus here.  The canonical way of building 
field lists in gigi is to prepend fields and call nreverse at the end.

-- 
Eric Botcazou