[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:

I guess https://github.com/llvm/llvm-project/pull/89441 is the real issue that https://github.com/llvm/llvm-project/pull/83660 caused, but we should still fix this for correctness, even though I'm not sure what the visible consequence is of this bug.

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