irinterp: Move irinterp va processing (#54076)
Currently we perform the va transform inside the innermost construct of
IRInterpretationState. However, I think it makes more sense to lift this
processing one level, so that the `argtypes` for the constructor that
takes `ir` matches `ir.argtypes`, while the one for the constructor that
takes `mi` is the full expanded out argtypes. NFC for base, but the `ir`
constructor is used in downstream external absints. However, I think
this way around is better for them also, since it's not always clear
what format the argtypes they have are in and the va direction is easy
and well supported, so it's better to have the option.
