[clang-doc] Make CommentInfo arena allocated
This patch move the CommentInfo type into the arena. It updates block
handling to collect child info types and serialize the array in one
shot.
We also clean up the test code to avoid using the arenas in the tests.
This has the upside of making the test more hermetic, and avoids churn
in the related code as the allocation API interfaces evolve.
Performance and memory usage regress slightly. This is somewhat expected
as we do not yet aggressively release short term memory during merge
operations. Future patches will reclaim this overhead.
| Metric | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| Time | 920.5s | 998.5s | 1010.5s | +9.8% | +1.2% |
| Memory | 86.0G | 43.8G | 47.8G | -44.4% | +9.2% |
| Benchmark | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| BM_BitcodeReader_Scale/10 | 67.9us | 68.8us | 68.7us | +1.1% | -0.2% |
| BM_BitcodeReader_Scale/10000 | 70.5ms | 24.5ms | 23.8ms | -66.2% | -2.8% |
| BM_BitcodeReader_Scale/4096 | 23.2ms | 4.4ms | 4.4ms | -80.9% | -0.0% |
| BM_BitcodeReader_Scale/512 | 509.4us | 538.6us | 538.3us | +5.7% | -0.1% |
| BM_BitcodeReader_Scale/64 | 114.8us | 116.9us | 116.8us | +1.8% | -0.1% |
| BM_Index_Insertion/10 | 2.3us | 4.2us | 3.8us | +65.6% | -8.2% |
| BM_Index_Insertion/10000 | 3.1ms | 5.3ms | 5.1ms | +61.7% | -5.4% |
| BM_Index_Insertion/4096 | 1.3ms | 2.1ms | 2.0ms | +56.8% | -6.1% |
| BM_Index_Insertion/512 | 153.6us | 257.2us | 243.3us | +58.5% | -5.4% |
| BM_Index_Insertion/64 | 18.1us | 30.5us | 29.1us | +61.1% | -4.8% |
| BM_JSONGenerator_Scale/10 | 36.8us | 36.6us | 37.6us | +2.2% | +2.9% |
| BM_JSONGenerator_Scale/10000 | 89.6ms | 90.2ms | 91.1ms | +1.6% | +1.0% |
| BM_JSONGenerator_Scale/4096 | 33.7ms | 34.5ms | 35.0ms | +3.7% | +1.3% |
| BM_JSONGenerator_Scale/512 | 1.9ms | 1.9ms | 2.0ms | +1.8% | +1.7% |
| BM_JSONGenerator_Scale/64 | 222.4us | 218.7us | 226.7us | +1.9% | +3.7% |
| BM_Mapper_Scale/10 | 2.5ms | 2.5ms | 2.5ms | +0.7% | +1.9% |
| BM_Mapper_Scale/10000 | 104.3ms | 106.1ms | 105.5ms | +1.1% | -0.6% |
| BM_Mapper_Scale/4096 | 44.3ms | 45.0ms | 44.5ms | +0.3% | -1.1% |
| BM_MergeInfos_Scale/10000 | 12.2ms | 2.0ms | 1.9ms | -84.8% | -7.1% |
| BM_MergeInfos_Scale/2 | 1.9us | 1.8us | 1.7us | -7.9% | -2.1% |
| BM_MergeInfos_Scale/4096 | 2.8ms | 574.7us | 531.2us | -80.9% | -7.6% |
| BM_MergeInfos_Scale/512 | 68.9us | 39.6us | 36.5us | -47.0% | -7.9% |
| BM_MergeInfos_Scale/64 | 10.3us | 6.5us | 6.0us | -41.3% | -7.0% |
| BM_MergeInfos_Scale/8 | 2.8us | 2.2us | 2.2us | -22.4% | -2.8% |
