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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> What's worse, it really depends on whether any `FileID` used by that file was ever deserialized

That reminds me of the issue @alexfh and I discussed here:
* [[clang][modules] NFCI: Pragma diagnostic mappings: write/read FileID instead of SourceLocation](https://reviews.llvm.org/D137213)
* https://github.com/llvm/llvm-project/pull/87427
* https://github.com/llvm/llvm-project/pull/87442

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