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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> Back to the original issue.

Do you control the module map that specifies a header as private? Does it need to stay private?

Personally, to me it looks like a bug that a private header is getting picked up [somewhat accidentally]. I'm not planning to fix this bug, just that relying on outright buggy behavior doesn't look promising long-term.

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