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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Interesting, changing `private header "wrap_foo.h"` to `header "wrap_foo.h"` in `b.wrap_foo` stops reproducing the error. I'm not sure it guarantees 
> whenever resolving module for the header, pick one that has the header as modular over textual.

But seems we are closer to this behavior than I expected.

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