[llvm-exegesis] Refactor common parts out of FunctionExecutorImpl

This patch refactors some code out of FunctionExecutorImpl into the base
class that should be common across all implementations of
FunctionExecutor. Particularly, this patch factors out
accumulateCounterValues, and also factors out runAndSample, moving
implementation specific code into a new runWithCounter function. This
makes adding new implementations of FunctinExecutor easier.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D148079