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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> @vsapsai I guess, it's a good sign? :) Do you see how our use case can be supported by a trivial and low-risk forward fix? If not, I'd insist on a revert before we can figure out the way forward. We can run this sort of a change through our testing before it relands, and ensure it doesn't break our code. We did this on multiple occasions before (especially for changes that touch Clang header modules).

I still don't understand what is your use case. My [very ungenerous] interpretation is "to access a file you were told not to access without triggering any tripwires". And I'm not sure such a use case is worth supporting.

But I understand that is only my own interpretation which can be incorrect. And I want to believe you have a better use case that doesn't rely on accessing private headers.

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