Clean up AMDGPU kernel attribute spelling and diagnostics

AMDGPU kernel attributes have some common cleanup work.

Today these attributes generally follow the existing AMDGPU pattern:

- GNU spelling works in C and C++.
- `[[clang::...]]` spelling works in C++, but not in C23.
- The attributes are accepted on non-AMDGPU targets and then ignored.
- Some kernel-only checks are handled in a manual Sema chain.

This is confusing because the GNU spelling already gives the attributes C semantics, so the C23 `[[clang::...]]` spelling should probably work too. Also, Clang generally diagnoses ignored attributes when an attribute has no effect.

This issue tracks follow-up cleanup for AMDGPU kernel attributes:

- Support C23 `[[clang::...]]` spelling where the GNU spelling already works in C.
- Diagnose AMDGPU-only attributes as ignored when used on targets where they have no effect.
- If feasible, move the repeated kernel-only AMDGPU attribute checks out of the manual Sema chain into a shared mechanism.

This was discussed in review for #211059.