[clang-doc] Support deep copy between arenas for merging
Upcoming changes to the merge step will necessitate that we clear the
transient arenas and merge new items into the persistent arena. However
there are some challenges with that, as the existing types typically
don't want to be copied. We introduce some new APIs to simplify that
task and ensure we don't accidentally leak memory.
On the performance front, we reclaim about 2% of the overhead, bringing
the cumulative overhead from the series of patches down to about 7% over
the baseline.
| Metric | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| Time | 920.5s | 1014.5s | 991.5s | +7.7% | -2.3% |
| Memory | 86.0G | 39.9G | 40.0G | -53.4% | +0.3% |
| Benchmark | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| BM_BitcodeReader_Scale/10 | 67.9us | 73.0us | 72.2us | +6.3% | -1.1% |
| BM_BitcodeReader_Scale/10000 | 70.5ms | 17.1ms | 22.5ms | -68.1% | +31.6% |
| BM_BitcodeReader_Scale/4096 | 23.2ms | 6.5ms | 6.6ms | -71.6% | +1.1% |
| BM_BitcodeReader_Scale/512 | 509.4us | 850.1us | 898.7us | +76.4% | +5.7% |
| BM_BitcodeReader_Scale/64 | 114.8us | 140.6us | 133.7us | +16.4% | -5.0% |
| BM_EmitInfoFunction | 1.6us | 1.8us | 1.9us | +16.8% | +3.4% |
| BM_Index_Insertion/10 | 2.3us | 4.1us | 4.1us | +78.7% | -0.5% |
| BM_Index_Insertion/10000 | 3.1ms | 5.4ms | 5.3ms | +70.5% | -0.4% |
| BM_Index_Insertion/4096 | 1.3ms | 2.1ms | 2.1ms | +66.0% | +0.7% |
| BM_Index_Insertion/512 | 153.6us | 252.9us | 251.8us | +64.0% | -0.4% |
| BM_Index_Insertion/64 | 18.1us | 30.4us | 30.2us | +67.4% | -0.5% |
| BM_JSONGenerator_Scale/10 | 36.8us | 38.1us | 37.1us | +0.6% | -2.8% |
| BM_JSONGenerator_Scale/10000 | 89.6ms | 84.2ms | 81.4ms | -9.1% | -3.3% |
| BM_JSONGenerator_Scale/4096 | 33.7ms | 32.1ms | 31.0ms | -8.1% | -3.6% |
| BM_JSONGenerator_Scale/512 | 1.9ms | 2.0ms | 1.9ms | -0.1% | -3.1% |
| BM_JSONGenerator_Scale/64 | 222.4us | 232.5us | 222.9us | +0.2% | -4.1% |
| BM_Mapper_Scale/10 | 2.5ms | 2.5ms | 2.5ms | -1.0% | -0.4% |
| BM_Mapper_Scale/10000 | 104.3ms | 102.1ms | 112.3ms | +7.7% | +10.0% |
| BM_Mapper_Scale/4096 | 44.3ms | 46.5ms | 45.0ms | +1.4% | -3.4% |
| BM_Mapper_Scale/512 | 7.6ms | 7.4ms | 7.7ms | +1.1% | +3.3% |
| BM_Mapper_Scale/64 | 3.1ms | 3.0ms | 3.0ms | -1.5% | -0.2% |
| BM_MergeInfos_Scale/10000 | 12.2ms | 931.2us | 575.6us | -95.3% | -38.2% |
| BM_MergeInfos_Scale/2 | 1.9us | 1.8us | 1.8us | -2.7% | +1.9% |
| BM_MergeInfos_Scale/4096 | 2.8ms | 201.3us | 205.3us | -92.6% | +1.9% |
| BM_MergeInfos_Scale/512 | 68.9us | 19.9us | 20.5us | -70.2% | +3.4% |
| BM_MergeInfos_Scale/64 | 10.3us | 4.1us | 3.8us | -62.6% | -7.1% |
| BM_MergeInfos_Scale/8 | 2.8us | 1.9us | 1.9us | -30.5% | +4.2% |
| BM_SerializeFunctionInfo | 25.5us | 26.2us | 25.8us | +0.9% | -1.5% |
