Determine which VC++ warning removal pragmas we don't need

[danluu]

This code was originally written for an old version of VC++ which had a lot more quirks than 2015. Some of the warning removal pragmas can probably be removed, and many of the ones that can't be removed should probably have better documentation.

It shouldn't be that hard to write a script that removes the pragmas one-by-one to see which ones are no longer necessary.

Also, in some cases, we might be able to just change the code to work cross-platform without some kind of platform-specific #ifdef'd pragma.

[danluu]

I was thinking about doing this but we have 7 outstanding PRs for code review and it's basically guaranteed that this will conflict with at least one of those when we try to merge.

[danluu]

I send out a codeflow for review for this the other day. It's pretty trivial and I'll merge it one it passes code review.