[trace][intel pt] Support dumping the trace info in json

Thanks to ymeng@fb.com for coming up with this change.

`thread trace dump info` can dump some metrics that can be useful for
analyzing the performance and quality of a trace. This diff adds a --json
option for dumping this information in json format that can be easily
understood my machines.

Differential Revision: https://reviews.llvm.org/D129332