Revert "[C++11] Replace LLVM atomics with std::atomic."

Breaks the MSVC build.
DataStream.cpp(44): error C2552: 'llvm::Statistic::Value' : non-aggregates cannot be initialized with initializer list

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202731 91177308-0d34-0410-b5e6-96231b3b80d8