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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Ok, I'm going to revert the change to help you out. But I'm going to re-land it in a week or when you are ready, whichever comes first.

There was no indication there is anything wrong with the change or if the issue is wide-spread. And if a single company relies on an existing side effect, there is no obligation for everyone else [to maintain such a side effect](https://xkcd.com/1172/) forever.

I'm willing to help figure out how to achieve the desired result in a different way. But for that need to know what is the desired result.

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