[dfsan] Conservative solution to atomic load/store

DFSan at store does store shadow data; store app data; and at load does
load shadow data; load app data.

When an application data is atomic, one overtainting case is

thread A: load shadow
thread B: store shadow
thread B: store app
thread A: load app

If the application address had been used by other flows, thread A reads
previous shadow, causing overtainting.

The change is similar to MSan's solution.
1) enforce ordering of app load/store
2) load shadow after load app; store shadow before shadow app
3) do not track atomic store by reseting its shadow to be 0.
The last one is to address a case like this.

Thread A: load app
Thread B: store shadow
Thread A: load shadow
Thread B: store app

This approach eliminates overtainting as a trade-off between undertainting
flows via shadow data race.

Note that this change addresses only native atomic instructions, but
does not support builtin libcalls yet.
   https://llvm.org/docs/Atomics.html#libcalls-atomic

Reviewed-by: morehouse

Differential Revision: https://reviews.llvm.org/D97310