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

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> And, in the end, the folks debugging this (Ilya & co) are busy working on clang header module reproduction and reduction tools: https://github.com/emaxx-google/cvise/branches @emaxx-google

Interestingly enough, @vsapsai is also working on reproducers for explicitly built clang modules, and that is how he ran into the compiler bug about a not-included textual header. When discovering which files to capture for the reproduction, we technically shouldn't need to also include this header input since it's not needed to build the module, it seems like that principle would also help for reducing reproducers.

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