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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> I know what we do is cheesy and we might need to reconsider how to approach it, but this is a breaking change and it would be great to find a resolution for it before committing to the new behavior. Could we revert this while we discuss the potential ways out of it?

What is your timeframe for stopping putting the same header ("wrap_foo.h") into multiple modules? If you do that it was never guaranteed which module would be used for a header. I'm willing to unblock you but I'd like to know if you are committed to fixing the issue in the module maps.

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