Pattern: Unordered include
Issue: -
We want to ensure that headers appear in the right order: 1) for foo.cc, foo.h (preferred location) 2) c system files 3) cpp system files 4) for foo.cc, foo.h (deprecated location) 5) other headers
We classify each include statement as one of those 5 types using a number of techniques. The include_state object keeps track of the highest type seen, and complains if we see a lower type after that.