Switch statistics to use relaxed updates to a std::atomic.

This removes usage of the hacky, incorrect, and TSan-unfriendly
home-grown atomics. It should actually be more efficient in some cases.

Based on our existing usage of <atomic>, all of this is portably
available AFAICT. One small challenge is initializing the stastic, but
I've tried a comparable sample out on MSVC (the most likely to complain
here) and it seems to work. Will have to watch the build bots of course.

llvm-svn: 271504