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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> 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 you're proposing is regression testing, which is (I hope) different from what we do today, because my understanding is that we don't just write tests and then put whatever `expected` directives needed to make them pass, but we actively consider whether it's correct to have diagnostics where they are issued (and, conversely, whether some examples ought to be accepted), and whether issued diagnsotics make sense.

I'd be opposed moving towards regression testing. From what I've heard how LLVM is tested these days, I don't want Clang to end up there. Even if we do, this won't apply to C++ DR tests anyway.

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

I don't think this work precludes from going in the direction of regression tests. Every test that passes with `-verify-directives` passes without it (kinda by definition), so if we find in the future that it gets in the way, we can easily drop it.

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