[ADT] Replace sys::MemoryFence with standard atomics.

This is a bit faster in theory, in practice it's cold code that's only
active in !NDEBUG, so it probably doesn't make a difference. This is one
of the last users of our homegrown Atomic.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323999 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed