[DebugCounters] Keep track of total counts

This patch makes debug counters keep track of the total number of times
we've called `shouldExecute` for each counter, so it's easier to build
automated tooling on top of these.

A patch to print these counts is coming soon.

Patch by Zhizhou Yang!

Differential Revision: https://reviews.llvm.org/D49560

llvm-svn: 337748