[clang] [clang][modules] HeaderSearch::MarkFileModuleHeader sets textual headers' HeaderFileInfo non-external when it shouldn't (PR #89005)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> From Jan:
> 
> > Maybe we can add the number of external HeaderFileInfos to HeaderSearch::PrintStats() and have a test that checks for it.
> > The flag to enable that output is -show-stats I think.

We tried this, but it seems that by the time the modules compile, everything ends up being local instead of external anyway. I think a unit test is probably good enough for this one since we aren't even sure what the practical consequence is of this bug.

https://github.com/llvm/llvm-project/pull/89005