[clang][Modules] Adding C-API for Negative Stat Caching Diagnostics

[qiongsiwu]

llvm#135703 added a C++ API to the shared cached to diagnose invalid negatively stat cached paths. This PR adds a C API so an external system can take advantage of the diagnostics.

[qiongsiwu]

Note to reviewers:
The test added here is simply testing that the API is called correctly. llvm#135703 has a test to test for correctly producing the paths. I am not aware of a simple way to test the C-API for its content (i.e. creating a scan where some file s created in the middle to invalidate the negative stat cache). If you are aware of such tests, I am all ears.

Thanks!

[jansvoboda11]

Only now do I see that we use CXCStringArray everywhere in the scanner API. This means that introducing new CXStringSet type will impose additional burden on the client. Unlike upstream, we already have factory functions that create reference-like CXCStringArray here in the next branch. Can we switch to that type instead?

[qiongsiwu]

Sounds good! The code is updated. Thanks for taking a careful look.

I don't particularly like the code I have for createCStringsRefImpl. It feels clumsy. I have to have the lambda to handle the simple task of getting the start of the string data. I am all ears if there are simpler ways to do this.