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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

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

I'm not sure yet, I think we'll need to investigate [Sam's reproducer](https://github.com/llvm/llvm-project/pull/89005#issuecomment-2067300965) and consider [Richard's comment](https://github.com/llvm/llvm-project/pull/83660#discussion_r1568027778). It's possible that we now really do end up with header file infos for textual headers where we didn't before, which triggers the virality much more often.

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