[llvm-branch-commits] [BOLT] Buffer DataAggregator diagnostics (PR #203464)

[Amir Ayupov via llvm-branch-commits]

https://github.com/aaupov created https://github.com/llvm/llvm-project/pull/203464

To avoid mixed up error messages in multi-perf case, provide diagnostics
buffer and stream for each aggregator job.

Test Plan: updated pre-aggregated-perf.test