This adds a PP callback for the has_include and has_include_next directives.
Checking for the presence of a header should add it to the list of header dependencies so this overrides the callback in the dependency scanner.
I kept the callback intentionally simple for now. If we want to extend it to track things like missing headers, just as PP->FileNotFound() does today, then that wouldn't be hard to add later.
This callback seems pretty unhelpful in the case where lookup of the file failed (you just get a source location). Could we pass in the FileName and IsAngled flag too (like we do for InclusionDirective)? I think that's what (eg) a callback tracking anti-dependencies (for a more sophisticated build system that can handle them) would want.