Implement DocumentSnapshot tracking in VS 4 Mac

[NTaylorMullen]

When we eventually move all of our internal parsing infrastructure to use DocumentSnapshots we'll need to have the correct information flowing through the VS4Mac side of the world.

[mkArtakMSFT]

@NTaylorMullen, how time critical is this? Is this necessary to be done in 2.2?

[NTaylorMullen]

@NTaylorMullen, how time critical is this? Is this necessary to be done in 2.2?

This may or may not fall in the 2.2 time frame. It all depends on when we want to refresh bits on the VS4Mac side of the world (as of right now, there's no feature requiring us to)