[trace][intelpt] Support system-wide tracing [17] - Some improvements

This improves several things and addresses comments up to the diff [11] in this stack.

- Simplify many functions to receive less parameters that they can identify easily
- Create Storage classes for Trace and TraceIntelPT that can make it easier to reason about what can change with live process refreshes and what cannot.
- Don't cache the perf zero conversion numbers in lldb-server to make sure we get the most up-to-date numbers.
- Move the thread identifaction from context switches to the bundle parser, to leave TraceIntelPT simpler. This also makes sure that the constructor of TraceIntelPT is invoked when the entire data has been checked to be correct.
- Normalize all bundle paths before the Processes, Threads and Modules are created, so that they can assume that all paths are correct and absolute
- Fix some issues in the tests. Now they all pass.
- return the specific instance when constructing PerThread and MultiCore processor tracers.
- Properly implement IntelPTMultiCoreTrace::TraceStart.
- Improve some comments.
- Use the typedef ContextSwitchTrace more often for clarity.
- Move CreateContextSwitchTracePerfEvent to Perf.h as a utility function.
- Synchronize better the state of the context switch and the intel pt
perf events.
- Use a booblean instead of an enum for the PerfEvent state.

Differential Revision: https://reviews.llvm.org/D127456