[clang] [clang][modules] Don't associate a header with an unresolved submodule (PR #225108)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

I don't think that preventing the crash while leaving the submodule table broken is the right fix. Doesn't look like the patch addresses the root cause. And if clang is in inconsistent state, it is better to fail early instead of trying to continue and failing in unpredictable way in unpredictable place.

https://github.com/llvm/llvm-project/pull/225108