PR19388: DebugInfo: Emit dead arguments in their originally declared order.

Unused arguments were not being added to the argument list, but instead
treated as arbitrary scope variables. This meant they weren't carefully
added in the original argument order.

In this particular example, though, it turns out the argument is only
/mostly/ unused (well, actually it's entirely used, but in a specific
way). It's a struct that, due to ABI reasons, is decomposed into chunks
(exactly one chunk, since it has one member) and then passed. Since only
one of those chunks is used (SROA, etc, kill the original reconstitution
code) we don't have a location to describe the whole variable.

In this particular case, since the struct consists of just the one int,
once we have partial location information, this should have a location
that describes the entire variable (since the piece is the entirety of
the object).

And at some point we'll need to describe the location of even /entirely/
unused arguments so that they can at least be printed on function entry.