[clang][DependencyScanning] Check for the Existence of CIWithContext before Finalizing

[qiongsiwu]

The by-name lookup APIs are intended for Swift. swiftlang/swift#85555 teaches Swift to use it. The original intention was to invoke the finalization API explicitly. swiftlang/swift#85555 folds the finalization step into the module dependency scanner's destructor. This leads to a case where if an initialization error occurs, the CIWithContext may not be setup for some workers, and calling the finalization API is incorrect.

This PR checks against the existence of the CIWithContext and then performs the finalization.

[qiongsiwu]

Note to reviewers:

1. I don't think the current API design fits well into the use case (especially being called in the destructor of its users). I'd like to redesign the API for finalization, but I feel like that may be too much for this PR and its twin [DependencyScanning] Perform Clang Module By-Name Dependency Query Using a Shared Clang Compiler Instance swift#85555. I plan to come back to this once these two PRs land.
2. We have test coverage for Swift https://github.com/swiftlang/swift/blob/6b3e5613203007b9a0494082b9046736e2a008b4/unittests/DependencyScan/ModuleDeps.cpp#L447 for exactly this case. That's how I discovered the problem. I will add a similar unit test case on the clang side when I refactor the API.

[qiongsiwu]

Please test with following PR:
swiftlang/swift#85555

@swift-ci please test

[jansvoboda11]

Makes sense. I agree that we should make the API more ergonomic.

[qiongsiwu]

Please test with following PR:
swiftlang/swift#85555

@swift-ci please test