Make mangled name available to `generated_xxx_override`. Link name override no longer affected by generated name override.

[Molot2032]

Would fix #3107

• The name passed to generated_link_name_override is no longer first modified by generated_name_override
• Makes the mangled name for items available in generated_link_name_override and generated_name_override, by including it as a field on the passed ItemInfo parameter.

[Molot2032]

Kindly requesting review @emilio @pvdrz

[emilio]

I think this is sensible, though it needs a rebase and it'd be good to document what the intended use case for having the mangling name around is.