Previously, we would drop uninhabited captures from lambda sets' runtime
representations, which meant sets like
```
[L1 {a: Str}, L2 {a: []}]
```
had runtime representation
```
{Str}
```
rather than
```
Union({Str}, {[]})
```
if we drop unreachable lambdas from the representation, then the
reachable lambdas are somewhat more efficient to compile (as there are
less material tag options), but the compiler complexity increases
because we must represent voided capture sets in the lambda set.
Even if a lambda has voided captures, we must specialize it, because
failing to do so opens us up to losing relevant specializations needed
later on. See 2f7020aa31 for a
previous occurence of that.
As such, simply keep voided layouts in place during lambda set
compilation. The optimizer should elide them anyway.
