[llvm] [ctx_prof] Move the "from json" logic more centrally to reuse it from test. (PR #106129)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Aug 27, 6:37 PM EDT**: @mtrofin started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/106129).


https://github.com/llvm/llvm-project/pull/106129