[clang] [clang] Use SmallString in CommentBriefParser to reduce heap allocs (PR #147853)

[Bogdan Vetrenko via cfe-commits]

b0gdnv wrote:

Thanks for your review. Yes, I ran micro-benchmarks to check performance implications.

### Benchmark results (mean)

| Benchmark         | Metric       | New        | Baseline   | Δ (%)   |
|------------------|--------------|------------|------------|---------|
| BM_Parse_Tiny     | CPU Time     | 494.7 ns   | 504.8 ns   | +2.0% (better) |
|                  | Throughput   | 196.1 MB/s | 192.2 MB/s | -2.0% (better) |
| BM_Parse_Overflow | CPU Time     | 650.5 ns   | 688.0 ns   | -5.5% (better) |
|                  | Throughput   | 213.8 MB/s | 202.0 MB/s | +5.8% (better) |
| BM_Parse_1K       | CPU Time     | 2194.5 ns  | 2289.0 ns  | -4.1% (better) |
|                  | Throughput   | 470.3 MB/s | 450.9 MB/s | +4.3% (better) |
| BM_Parse_4K       | CPU Time     | 7146.3 ns  | 7321.4 ns  | -2.4% (better) |
|                  | Throughput   | 573.8 MB/s | 560.0 MB/s | +2.5% (better) |
| BM_Parse_8K       | CPU Time     | 13592.7 ns | 13994.3 ns | -2.9% (better) |
|                  | Throughput   | 605.5 MB/s | 588.2 MB/s | +2.9% (better) |
| BM_Parse_32K      | CPU Time     | 53524.0 ns | 53229.9 ns | -0.6% (worse) |
|                  | Throughput   | 612.6 MB/s | 615.7 MB/s | +0.5% (worse) |

You can find the benchmark source code here (not intended for commit):  
https://gist.github.com/b0gdnv/fb6b33eb2129747cb0030abdfba3b2b3

Raw JSON results:  
[brief_new.json](https://github.com/user-attachments/files/21163591/brief_new.json)  
[brief_cur.json](https://github.com/user-attachments/files/21163593/brief_cur.json)

Let me know if you'd prefer this benchmark included somewhere more formal.

https://github.com/llvm/llvm-project/pull/147853