InternPool: eliminate `var_args_param_type`

[mlugg]

This was a "fake" type used to handle C varargs parameters, much like generic poison. In fact, it is treated identically to generic poison in all cases other than one (the final coercion of a call argument), which is trivially special-cased. Thus, it makes sense to remove this special tag and instead use generic_poison_type in its place. This fixes several bugs in Sema related to missing handling of this tag.

Resolves: #19781