[llvm-exegesis] Add ability to assign perf counters to specific PID

This patch gives the ability to assign performance counters within
llvm-exegesis to a specific process by passing its PID. This is needed
later on for implementing a subprocess executor. Defaults to zero, the
current process, for the InProcessFunctionExecutorImpl.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D151020