[trace][intelpt] Support system-wide tracing [16] - Create threads...

[trace][intelpt] Support system-wide tracing [16] - Create threads automatically from context switch data in the post-mortem case

For some context, The context switch data contains information of which threads were
executed by each traced process, therefore it's not necessary to specify
them in the trace file.

So this diffs adds support for that automatic feature. Eventually we
could include it to live processes as well.

Differential Revision: https://reviews.llvm.org/D127001