[llvm] [lit] Migrate lit to ThreadPoolExecutor (PR #212397)

[Prasoon Kumar via llvm-commits]

prasoon054 wrote:

Performance measurement (macOS 26.5.2, M5)

#### check-llvm

| Backend | Time |
|---|---|
| PPE | 216s |
| TPE | 401s (~1.86x) |

#### llvm-mca: `-j10`

| Backend | Time |
|---|---|
| PPE | 3.16s |
| TPE | 5.49s (~1.74x) |

#### llvm-mca: Scalene breakdown across `-j`

| `-j` | Testing time | python % | C/native % | sys % |
|---|---|---|---|---|
| 1 | 12.77s | 3.62% | 10.28% | 83.49% |
| 10 | 6.10s | 2.90% | **34.12%** | 62.20% |

#### C / native % by file, `-j1` vs `-j10`

| File | C% at `-j1` | C% at `-j10` |
|---|---|---|
| `posixpath.py` | 0.23 | 10.05 |
| `threading.py` | 7.28 | 7.06 |
| `subprocess.py` | 1.15 | 5.67 |
| `TestRunner.py` (ours) | 0.23 | 4.18 |
| `tempfile.py` | 0.00 | 2.60 |

https://github.com/llvm/llvm-project/pull/212397