[clang] [clang-tools-extra] [clang][include cleaner] Fix MainHeader insertion issue (PR #212852)

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet approved this pull request.

thanks, LGTM! but i'd wait for @HazardyKnusperkeks's review as well, especially as this is touching clang-format pieces now.

https://github.com/llvm/llvm-project/pull/212852