Large number of clang warnings

[danluu]

We have so many warnings that if we enable general warnings the build errors out with "too many errors".

I've fixed some of these in 603d5fc and 5cc6904 but there are still a ton of these left.

[danluu]

I keep cleaning out the analogous warnings from BitFunnel, but now that we've integrated NativeJIT with BitFunnel it's going to be hard to stay warning clean without fixing these.

[danluu]

The next obvious warning to clean out is the precision loss / sign extension stuff. The last time I went through those I made some change that caused a subtle error that caused tests to fail. I don't really want to try this again unless our tests are clean and our tests have been failing on BitFunnel/BitFunnel#298 since Nov 2016. @MikeHopcroft has a proposed fix for this but I don't know what it is so this issue is blocked on that.