[clang-tools-extra] [clangd] fix crash in include cleaner (PR #99514)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Having debugged the failing scenario a bit (see [this comment](https://github.com/llvm/llvm-project/issues/99617#issuecomment-2309220224)), my sense is that we do not want to accept `MacroRefs` entries which are not in the main file; instead, I have a proposal for an alternative solution at the end of that comment.

Let me know if you're interested in implementing it, otherwise I can do it.

https://github.com/llvm/llvm-project/pull/99514