[clang] [clang][test] Add `%clang_cc1_arm64_neon` substitution (PR #188547)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > So there is no Windows-specific testing needed?
> 
> There is currently much less Windows-specific coverage in this area, so I think it is reasonable to keep those tests separate for now.

Good to know, thanks! I suppose that's fine, but I do worry that we'll then end up with three more substitutions later for Windows environments, too.

> > -Wall enables a lot of diagnostics, including ones that are likely to be unrelated to what the test file is after. -Werror in a test file is definitely odd as well -- why do we need the diagnostics to be errors, is there something about that code path which could impact test behavior?
> 
> As far as I can tell, `-Wall`, `-Werror`, and `-flax-vector-conversions={...`} are purely Sema-related flags, so I also would not expect them to be part of codegen tests. My guess is that, given the volume of these tests, they were included to catch subtle Sema issues as well. I do see some value in that, but if we want to keep that coverage, I think it would be better to hide those behind something like `%sema_flags`.

FWIW, `-Wall` flags are not purely sema related but are mostly sema related. e.g., we do have diagnostics which we emit only at codegen time, but I'd expect those tests to pass `-verify` in those cases, and I wouldn't expect `-Werror`. `-flax-vector-conversions` is handled mostly in Sema but changes which implicit conversions happen and so I can sort of see it as a codegen option but I think we could test that without requiring codegen tests (e.g., we could write tests which match the AST as opposed to codegen, if we wanted).

I'm not convinced those flags are so problematic in the RUN line that it's better to hide them from people reading the test.

https://github.com/llvm/llvm-project/pull/188547