[llvm] [InstrRef][nfc] Remove usage of unique_ptrs of arrays (PR #74203)

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

> FYI this seems to significantly increase memory usage: http://llvm-compile-time-tracker.com/compare.php?from=0d02ecc6bddc5e8c8bacbae3ab72a10467e177b4&to=1b531d54f6234488dec048fec1c5aca331bf8f3d&stat=max-rss&linkStats=on (look at `stage1-ReleaseLTO-g (link only)` only). mafft and SPASS go up by 10%.

How is the compiler itself built for this? IIUC the configurations refer to how the _tests_ are built, but not how the compiler itself is built.

Also, the regressions seem to happen in the link stage?

```
SPASS.link	288MiB	319MiB (+10.88%)
pairlocalalign.link	119MiB	131MiB (+10.41%)
```

https://github.com/llvm/llvm-project/pull/74203