[trace][intelpt] Support system-wide tracing [1] - Add a method for accessing...

[trace][intelpt] Support system-wide tracing [1] - Add a method for accessing the list of logical core ids

In order to open perf events per core, we need to first get the list of
core ids available in the system. So I'm adding a function that does
that by parsing /proc/cpuinfo. That seems to be the simplest and most
portable way to do that.

Besides that, I made a few refactors and renames to reflect better that
the cpu info that we use in lldb-server comes from procfs.

Differential Revision: https://reviews.llvm.org/D124573