[clang] [Modules] Don't fail when an unused textual header is missing. (PR #138227)

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

> I don't think we can ever get rid of that pattern and we will keep relying on Clang supporting this.

> We would need to example I shared above to keep working. 

I don't have a strong opinion on the best path forward here, but I'm not clear if you're actually willing to 

https://github.com/llvm/llvm-project/pull/138227