[trace][intelpt] Support system-wide tracing [19] - Some other minor improvements

This addresses the issues in diffs [13], [14] and [16]

- Add better documentation
- Fix some castings by making them safer
- Simplify CorrelateContextSwitchesAndIntelPtTraces
- Rename some functions

Differential Revision: https://reviews.llvm.org/D127804