[clang-doc] Migrate Namespaces to arena allocation (#190048)

This patch allocates the NamespaceInfo types in the local arenas, and
adapts the merging logic for the new list type and its children.
Memory use and performance improve slightly. Micro-benchmarks show a
regression in merge operations due to the more complex list operations.

 ## Build Clang-Doc Documentation
| Metric | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| Time | 920.5s | 1009.2s | 1002.4s | +8.9% | -0.7% |
| Memory | 86.0G | 43.2G | 43.9G | -49.0% | +1.6% |

 ## Microbenchmarks (Filtered for >1% Delta)
| Benchmark | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| BM_BitcodeReader_Scale/10 | 67.9us | 69.7us | 69.3us | +1.9% | -0.7% |
| BM_BitcodeReader_Scale/10000 | 70.5ms | 22.3ms | 24.8ms | -64.8% |
+11.4% |
| BM_BitcodeReader_Scale/4096 | 23.2ms | 4.7ms | 4.4ms | -80.9% | -5.7%
|
| BM_BitcodeReader_Scale/512 | 509.4us | 558.7us | 540.2us | +6.0% |
-3.3% |
| BM_BitcodeReader_Scale/64 | 114.8us | 119.2us | 117.0us | +1.9% |
-1.8% |
| BM_EmitInfoFunction | 1.6us | 1.6us | 1.6us | -1.1% | +0.8% |
| BM_Index_Insertion/10 | 2.3us | 4.2us | 3.7us | +61.7% | -11.3% |
| BM_Index_Insertion/10000 | 3.1ms | 5.4ms | 4.9ms | +56.8% | -9.1% |
| BM_Index_Insertion/4096 | 1.3ms | 2.2ms | 2.0ms | +54.5% | -8.2% |
| BM_Index_Insertion/512 | 153.6us | 259.6us | 236.7us | +54.1% | -8.8%
|
| BM_Index_Insertion/64 | 18.1us | 30.7us | 27.9us | +54.5% | -9.0% |
| BM_JSONGenerator_Scale/10 | 36.8us | 37.6us | 36.6us | -0.7% | -2.7% |
| BM_JSONGenerator_Scale/4096 | 33.7ms | 34.3ms | 34.2ms | +1.4% | -0.3%
|
| BM_JSONGenerator_Scale/64 | 222.4us | 225.6us | 220.2us | -1.0% |
-2.4% |
| BM_Mapper_Scale/10 | 2.5ms | 2.5ms | 2.5ms | -1.4% | -1.7% |
| BM_Mapper_Scale/10000 | 104.3ms | 109.3ms | 105.9ms | +1.6% | -3.1% |
| BM_Mapper_Scale/4096 | 44.3ms | 43.9ms | 44.7ms | +0.8% | +1.8% |
| BM_Mapper_Scale/512 | 7.6ms | 7.6ms | 7.6ms | +0.8% | +1.2% |
| BM_MergeInfos_Scale/10000 | 12.2ms | 1.6ms | 2.0ms | -83.6% | +28.7% |
| BM_MergeInfos_Scale/2 | 1.9us | 1.7us | 1.7us | -8.1% | +0.9% |
| BM_MergeInfos_Scale/4096 | 2.8ms | 520.5us | 557.4us | -79.9% | +7.1%
|
| BM_MergeInfos_Scale/512 | 68.9us | 37.9us | 39.9us | -42.0% | +5.3% |
| BM_MergeInfos_Scale/64 | 10.3us | 6.3us | 6.5us | -36.7% | +2.7% |
| BM_MergeInfos_Scale/8 | 2.8us | 2.2us | 2.2us | -20.7% | +1.2% |
| BM_SerializeFunctionInfo | 25.5us | 25.8us | 26.1us | +2.1% | +1.1% |

GitOrigin-RevId: 1deab59f6f82e75c6f3afd895a222e328ec51150