[trace][intelpt] Support system-wide tracing [3] - Refactor IntelPTThreadTrace

I'm refactoring IntelPTThreadTrace into IntelPTSingleBufferTrace so that it can
both single threads or single cores. In this diff I'm basically renaming the
class, moving it to its own file, and removing all the pieces that are not used
along with some basic cleanup.

Differential Revision: https://reviews.llvm.org/D124648