[trace][intel pt] Create a CPU change event and expose it in the dumper

Thanks to fredzhou@fb.com for coming up with this feature.

When tracing in per-cpu mode, we have information of in which cpu we are execution each instruction, which comes from the context switch trace. This diff makes this information available as a `cpu changed event`, which an additional accessor in the cursor `GetCPU()`. As cpu changes are very infrequent, any consumer should listen to cpu change events instead of querying the actual cpu of a trace item. Once a cpu change event is seen, the consumer can invoke GetCPU() to get that information. Also, it's possible to invoke GetCPU() on an arbitrary instruction item, which will return the last cpu seen. However, this call is O(logn) and should be used sparingly.

Manually tested with a sample program that starts on cpu 52, then goes to 18, and then goes back to 52.

Differential Revision: https://reviews.llvm.org/D129340