[llvm] [DWARFLinker] Walk each shared subtree's dependencies once (PR #218072)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

## Default threading (36 threads)

| Metric | Before | After | Change |
|---|---|---|---|
| Wall time | 24.83 s | 22.64 s | 1.10x faster |
| CPU time | 217.29 s | 67.92 s | 3.20x less |
| Peak RSS | 18.74 GiB | 5.94 GiB | 3.16x less |

## Single-threaded (`--num-threads 1`)

| Metric | Before | After | Change |
|---|---|---|---|
| Wall time | 169.28 s | 40.98 s | 4.13x faster |
| CPU time | 178.57 s | 51.06 s | 3.50x less |
| Peak RSS | 13.42 GiB | 4.86 GiB | 2.76x less |

https://github.com/llvm/llvm-project/pull/218072