As part of the Attributor manifest we want to change the signature of
functions. This patch introduces a fairly generic interface to do so.
As a first, very simple, use case, we remove unused arguments. A second
use case, pointer privatization, will be committed with this patch as
well.
A lot of the code and ideas are taken from argument promotion.
Replaced/Replacement is a bit confusing. Could you change to Old/New or more appropriate words?