run clang-tidy with readability-braces-around-statements

[greglandrum]

During the discussion of #2877 we talked about making a code formatting change to add braces for all if, for, etc. statements, including one liners. This is that change.

Along the way clang-tidy made a few additional mods to the code (using the same settings we used before).

I'm ambivalent about the formatting change and don't think it really adds that much, but I'm not opposed, so if the community think it makes sense, I'll go along. @d-b-w and @bp-kelley: you guys asked for this... think it's worth it?

Other operations here:

• clang-format the results
• clean up all the parts that clang-tidy-8 broke

[d-b-w]

I think that using braces around single lines and using nullptr are sufficiently nice that this would be a good change.

Also, there are a couple other clang-tidy fixers that I've found helpful, and so this makes diffs for applying those in the future smaller :)

(I meant this to be an approving review)

[greglandrum]

Ok, I'll go ahead and merge this. The longer we wait the more conflicts we'll see down the road. :-)