Summary:
This implementation should suffice for MSVC. _ReadWriteBarrier()
generates no instructions but instructs the compiler not to reorder accesses
around it, and we have to rely on MSVC not currently having any other
optimizations for const members to worry about.

Reviewed By: yfeldblum

Differential Revision: D5723782

fbshipit-source-id: a68caa4673a5c410b326c1d2e3c0752bd945f4a6