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

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

Firstly, thank you for your responsiveness @ilya-biryukov, and for helping us understand how this bug fix impacts Google's build. 

For all individuals involved in this discussion, I would like to remind that our [code of conduct](https://llvm.org/docs/CodeOfConduct.html) does ask us to understand each other when we disagree. 

In general, though, I share the same concern as @vsapsai about leaving this patch reverted for too long.  But in fear of speaking too much on behalf of others, perhaps it would be helpful for us to have a video call, coordinated by the area team, or otherwise, to understand how to move forward if we cannot come to a resolution soon.

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