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

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

Just looking at the `missing_textual_header` module alone this is a bit odd, but that's just the semantics of textual headers. Really textual headers just exist to control the non-modular include warning, so this is fine.

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