ctest: add suport for C enum

[mbyx]

Description

Adds the ability to specify whether a type alias and a set of constants that use that type alias maps to a C enum, as well as the ability to skip them.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[mbyx]

The signededness of a C enum is implementation defined, so should that test be skipped for all C enums?

[tgross35]

Could this take &Type rather than &str? To match rename_alias.

Maybe the name alias_is_c_enum would be a bit more descriptive, there are a handful of ways to represent C enums https://mdaverde.com/posts/rust-bindgen-enum/

[tgross35]

The signededness of a C enum is implementation defined, so should that test be skipped for all C enums?

repr(C) should generally match. I know there might be some problems with that, but have you run into any here?

Could you add a test in ctest-test to ensure this catches incorrect item values and things like the wrong repr type?

[mbyx]

If I use an unsigned type in simple.h for the enum, it fails on the signededness test on windows. If I use a signed type it fails on linux. This isn't really a problem in the actual ports because the windows port doesn't use enums, but these integration tests are run on all platforms.

[tgross35]

I think that's actually an important catch; the fallback here

libc/src/macros.rs

Line 279 in 2e670a8

(@ty) => { $crate::c_uint };

should probably be updated so the type is signed on Windows.