llvm-include-order check inconsistent with clang-format

[sam-mccall]

clang-format wants:

#include "aaaaa.h"
#include "zzzzz.h"
#include "gtest/gtest.h"
#include <algorithm>
#include <vector>

(gtest is a group above system headers)

clang-tidy wants:

#include "aaaaa.h"
#include "zzzzz.h"
#include <algorithm>
#include "gtest/gtest.h"
#include <vector>

(gtest is grouped within system headers)

I think this went wrong in 5a9f386

[llvmbot]

@llvm/issue-subscribers-clang-tidy