WIP: implement sret_union ABI for pointer-ful types

[topolarity]

This effectively expands our existing union ABI to cover both of these existing cases:

• sret ABI (which can stack-allocate a single pointer-ful type)
• union ABI (which can stack-allocate many pointer-free types)

This provides some nice speed-ups for temporary "wrappers":

const v = Any[]
@noinline maybe_wrapped(i) = (i % 32 != 0) ? Some(v) : nothing
function foo()
    count = 0
    for i = 1:1_000_000
        count += (maybe_wrapped(i) !== nothing) ? 1 : 0
    end
    return count
end

On this PR this gives:

julia> @btime foo()
  1.675 ms (0 allocations: 0 bytes)
968750

compared to current master:

julia> @btime foo()
  6.877 ms (968750 allocations: 14.78 MiB)
968750

TODO:

• Figure out what to do about PHI nodes
• Figure out why ScopedValue is allocating when accessed #53584 is not fixed, if not the ϕ-node issue

The most outstanding TODO here is what to do about ϕ-nodes. Right now, if the incoming Union{...} type has a pointer-containing type then this change forces the incoming object to be boxed, even if the object at run-time is actually pointer-free.

But that's just a band-aid so the code works - it introduces new boxes where we didn't have them before, which is a regression that almost certainly needs to be fixed before landing this.