[clang] [analyzer][docs] Document how to use perf and uftrace to debug performance issues (PR #126520)

[Arseniy Zaostrovnykh via cfe-commits]

https://github.com/necto commented:

One note about the illustrations:
Thank you for adding them, I think they are invaluable to understand the docs!
Yet, I feel they could be smaller both in detail and in the file sizes.
- Fewer details means the reader will quicker understand the points you want to make.
- Lower file sizes will save bandwidth and disk space to thousands of people and robots that checkout LLVM every day. Especially for such docs that are unlikely to be viewed by more than a hundred of people.

For example, here is how I minimized my PNG illustration using ImageMagick:
```
convert speedscope.png -resize 1825x900 +dither -colors 64 -strip -quality 90 -define png:compression-level=9 speedscope-low.png
```
I played with the resolution, colors, and quality parameters trying to find the smallest values that don't destroy the readability completely.


https://github.com/llvm/llvm-project/pull/126520