[clang] [clang][modules] Allow module maps with textual headers to be non-affecting (PR #89441)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> I updated the description of this PR, hopefully it makes more sense now. I still need to investigate what goes wrong in "Modules/preprocess-decluse.cpp". It seems that it assumes `%t/b.pcm` embeds the information from "a.modulemap".

Is this a pre-existing issue, or did my patch change to make "each textual header gets a `HFI`"?

https://github.com/llvm/llvm-project/pull/89441