Fold the switch statement to have each arm share as much code between them as possible. This reduces a ton of duplication from the error continuations and makes it slightly easier to update this code when more kinds of cases show up here later.
Annotate the covered switches with `llvm_unreachable` to avoid the MSVC
warning which does not recognise the covered switches. This allows us
to avoid a spew of warnings.
Request-based incremental dependencies are enabled by default. For the time being, add a flag that will turn them off and switch back to manual dependency tracking.
The DependencyVerifier is a DiagnosticVerifier-alike utility that takes annotations in user code and transforms them into a set of expectations. Those expectations are met by corresponding "obligations", which are constructed by reading the contents of the referenced name tracker associated with the source files for each of the primary inputs to a frontend job. The verifier will then pair off expectations and obligations, and ensure that any remaining unpaired obligations are diagnosed as failures.
This tool will not only ensure the correctness of the referenced name tracker's output, it will provide us with tests that ensure its output is stable, and give us the confidence to replace it with an evaluator-based tracking scheme in the future.
Resolves rdar://59773883
