[Dependency Scanning] Adding Diagnostics C-API for Invalid Negative-stat-cached Paths Incurred During Clang Modules Scan

[qiongsiwu]

This PR adds a C-API swiftscan_scanner_get_invalid_negative_stat_cached_paths that takes a
DependencyScanningTool as input.

rdar://150954767

[qiongsiwu]

Note to reviewers:

1. I am not sure if it is sufficient to simply pick the base file system to check against new files as I did in SwiftDependencyScanningService::getInvalidNegativeStatCachedPaths. Comments/suggestions are welcome!
2. Most of the PR is plumbing. Specifically, I need to propagate the information from the SharedCache within the DependencyScanningService all the way up to Swift's DependencyScanningService. Additionally, a new function swiftscan_string_set_t * create_set(const std::vector<llvm::StringRef> &stringRefs) is added to avoid converting a vector of StringRefs to a vector of std::strings.
3. I don't have a good idea of how to add a unit test for this feature. The key challenge is that I need to do two scans with the same service, and insert a new file into the file system in between the scans. I am looking into ways to implement this test. Pointers to existing examples are appreciated!

[cachemeifyoucan]

Can you specify the intent usage this new API? In both commit message and proper documentation around the API?

For the test, I created swift-scan-test binary to test C APIs from scanner. There is a scan option but only takes one command. Potentially, you can add a new command that takes multiple commands (maybe through a YAML or JSON file) so some work needs to be done here.

Alternatively, you can look to write the test in swift-driver repo.

[cachemeifyoucan]

Maybe better to change to create_empty_set?

[cachemeifyoucan]

This should not be in the Scanner Diagnostics API section and need proper documentation.

[qiongsiwu]

I think I can test the C++ API in Swift's DependencyScanningService by adding a new test in https://github.com/swiftlang/swift/tree/main/unittests/DependencyScan. There I can have some control over the instance of scanning service and the sequences of actions we need to trigger the diagnostics (similar to https://github.com/swiftlang/swift/blob/main/unittests/DependencyScan/ModuleDeps.cpp).