Renamed ThreadIntelPT to ThreadTrace, making it a top-level class. I noticed that this class can and should work with any trace plugin and there's nothing intel-pt specific in it.
With that TraceThread change, I was able to move most of the json file parsing logic to the base class TraceSessionFileParser, which makes adding new plug-ins easier.
This originally was part of https://reviews.llvm.org/D89283
const FileSpec &, or just FileSpec might be better here.