[clang-doc] Move Info types into arenas (#190054)

Info types used to own significant chunks of data. As we move these into
local arenas, these types must be trivially destructible, to avoid
leaking resources when the arena is reset. Unfortunatly, there isn't a
good way to transition all the data types one at a time, since most of
them are tied together in some way. Further, as they're now allocated in
the arenas, they often cannot be treated the same way, and even the
aliases and interfaces put in place to simplify the transition cannot
cover the full range of changes required.

We also use some SFINAE tricks to avoid adding boilerplate for helper
APIs, we'd otherwise have to support

Though it introduces some additional churn, we also try to keep tests
from using arena allocation as much as possible, since this is not
required to test the implementation of the library. As much of the test
code needed to be rewritten anyway, we take the opportunity to
transition now.

We see a large improvement in overall memory use, dropping by 15% over
the previous patches, and taking the total savings down to almost 54%.
Performance numbers stay mostly flat, but merging is vastly improved,
seeing a reduction of 40+% over the previous patch, and a 90% reduction
overall.

| Metric | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| Time | 920.5s | 1013.6s | 1014.5s | +10.2% | +0.1% |
| Memory | 86.0G | 47.3G | 39.9G | -53.6% | -15.6% |

| Benchmark | Baseline | Prev | This | Culm% | Seq% |
| :--- | :--- | :--- | :--- | :--- | :--- |
| BM_BitcodeReader_Scale/10 | 67.9us | 69.1us | 73.0us | +7.5% | +5.8% |
| BM_BitcodeReader_Scale/10000 | 70.5ms | 24.5ms | 17.1ms | -75.8% |
-30.4% |
| BM_BitcodeReader_Scale/4096 | 23.2ms | 4.5ms | 6.5ms | -71.9% | +45.1%
|
| BM_BitcodeReader_Scale/512 | 509.4us | 540.3us | 850.1us | +66.9% |
+57.4% |
| BM_BitcodeReader_Scale/64 | 114.8us | 117.3us | 140.6us | +22.5% |
+19.9% |
| BM_EmitInfoFunction | 1.6us | 1.6us | 1.8us | +12.9% | +15.0% |
| BM_Index_Insertion/10 | 2.3us | 4.2us | 4.1us | +79.5% | -1.1% |
| BM_Index_Insertion/10000 | 3.1ms | 5.2ms | 5.4ms | +71.1% | +3.7% |
| BM_Index_Insertion/4096 | 1.3ms | 2.1ms | 2.1ms | +64.9% | +2.0% |
| BM_Index_Insertion/512 | 153.6us | 244.7us | 252.9us | +64.7% | +3.3%
|
| BM_Index_Insertion/64 | 18.1us | 29.1us | 30.4us | +68.3% | +4.4% |
| BM_JSONGenerator_Scale/10 | 36.8us | 38.3us | 38.1us | +3.5% | -0.4% |
| BM_JSONGenerator_Scale/10000 | 89.6ms | 90.8ms | 84.2ms | -6.0% |
-7.2% |
| BM_JSONGenerator_Scale/4096 | 33.7ms | 35.9ms | 32.1ms | -4.7% |
-10.5% |
| BM_JSONGenerator_Scale/512 | 1.9ms | 2.0ms | 2.0ms | +3.1% | +0.4% |
| BM_JSONGenerator_Scale/64 | 222.4us | 231.9us | 232.5us | +4.5% |
+0.2% |
| BM_Mapper_Scale/10000 | 104.3ms | 104.9ms | 102.1ms | -2.1% | -2.7% |
| BM_Mapper_Scale/4096 | 44.3ms | 44.4ms | 46.5ms | +4.9% | +4.9% |
| BM_Mapper_Scale/512 | 7.6ms | 7.6ms | 7.4ms | -2.1% | -1.8% |
| BM_Mapper_Scale/64 | 3.1ms | 3.0ms | 3.0ms | -1.3% | -0.0% |
| BM_MergeInfos_Scale/10000 | 12.2ms | 1.9ms | 931.2us | -92.4% | -49.8%
|
| BM_MergeInfos_Scale/2 | 1.9us | 1.7us | 1.8us | -4.5% | +4.7% |
| BM_MergeInfos_Scale/4096 | 2.8ms | 521.8us | 201.3us | -92.8% | -61.4%
|
| BM_MergeInfos_Scale/512 | 68.9us | 34.9us | 19.9us | -71.2% | -43.1% |
| BM_MergeInfos_Scale/64 | 10.3us | 5.8us | 4.1us | -59.8% | -28.4% |
| BM_MergeInfos_Scale/8 | 2.8us | 2.1us | 1.9us | -33.3% | -12.6% |
| BM_SerializeFunctionInfo | 25.5us | 26.0us | 26.2us | +2.4% | +0.8% |