React Compiler: Make AbstractValue Copy to reduce heap allocation (#37225)

## Summary

By replicating the `IndexSet` behavior with a simple inline array, this
is able to avoid any heap allocation / memory thrash for
`AbstractValue`. It also reduces the size of AbstractValue from 72 bytes
+ all of the heap allocations, to only 18 bytes.

## How did you test this change?

Ran all of the fixtures to confirm byte output is identical.

Effects on memory and compile time:

| Benchmark | Peak allocation | Allocation count | Wall time |

|------------------|-----------------------------|------------------|-----------|
| legacy/image.tsx | 58.21 -> 33.40 MiB (-42.6%) | -51.1% | -39.5% |
| next-client | 58.21 -> 33.40 (-42.6%) | -40.5% | -25.1% |
| devtools | 26.39 -> 16.29 (-38.3%) | -26.7% | -14.8% |
| fixtures | 17.35 → 9.41 (-45.8%) | -9.9% | -9.0% |