[clang] [llvm] [clang] Add `-verify-directives` cc1 flag (PR #179835)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Taking a step back, I think what we should be aiming for here is to, at some point, being able to autogenerate the diagnostics expectations for a test file, so that tests can be automatically updated.

What would the design for that look like? How does the direction this patch pushes us to, compare to that final direction?

https://github.com/llvm/llvm-project/pull/179835