Integrate IWYU

[goldsborough]

Putting this up as a "wishlist" issue: Since we already have clang-tidy, let's also integrate Include-What-You-Use. LLVM has a lot of different headers so having a tool to tell us what to include where will be healthy for the future and avoid things like cyclic dependencies or other badness that can arise from C++'s lack of a module system. It also avoid the kind of thing where you remove one header and the whole card-house of mismanaged includes breaks down. I managed to integrate it with my last LLVM project. It's excruciatingly slow, but would be great to have as a separate build rule to run over night or before lunch once in a while.