[llvm] [BOLT][Perf2bolt] Add support to generate pre-parsed perf data (PR #171144)

[Paschalis Mpeis via llvm-commits]

================
@@ -449,6 +469,51 @@ class DataAggregator : public DataReader {
   /// an external tool.
   std::error_code parsePreAggregatedLBRSamples();
 
+  /// Dump pre-parsed perf profile data into a single file.
+  /// The generator relies on the aggregator work to spawn the required
+  /// perf-script jobs based on the the aggregation type, and merges
+  /// their results into a single file.
+  /// This hybrid profile contains all required events such as BuildID,
+  /// MMAP, TASK, MAIN (brstack or basic samples), or MEM for the aggregation.
+  /// The generator also creates a file header, where these events
+  /// are listed along with the length information of their contents.
+  /// The given length numbers in the header are in bytes, they are used
+  /// as an offset int the pre-parsed profile.
+  /// Some of these events are essential to be presented in the file.
+  /// Please see a short summary below:
----------------
paschalis-mpeis wrote:

```suggestion
  /// Some of these events are required to be present in the file.
  /// 
  /// Short description of supported events:
```

https://github.com/llvm/llvm-project/pull/171144