[SanitizerBinaryMetadata] Pretend compiler-generated loads/stores are atomic

Profiling and GCOV generate known data-racy loads/stores. Pretend they
are atomic so that analysis using PC-keyed metadata for atomics do not
report them as data races (which would look like false positives).

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D142982