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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

We reviewed this thread today at the Clang Area Team meeting and it seems like y'all have perhaps found a solution everyone is happy with. However, if you'd like a meeting or if there's some other way we can support you, we're definitely happy to help!

> Please let me know if this is the right escalation path, if not, I am happy to find another one if this is wrong.

The usual path is: relevant maintainer(s), then lead maintainer, then relevant area team(s), then project council. But there's really no "wrong" group to escalate to, so this was a perfectly reasonable escalation path.

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