gh-101907: Removes use of non-standard C++ extension from Include/cpython/code.h

[zooba]

Updates all references to use the macros, so that changes are localised to the header file.

• Issue: code.h uses MS-only C++ extensions #101907

[zooba]

Another possible fix (that I haven't tested yet) would be to move this struct into a private header. I don't have any strong preference, but I'm also not familiar with its background and intended use.

[markshannon]

This is C11, not C++.

What problem is this causing? Looks like I should have read the issue.

[markshannon]

Anonymous unions are quite useful, especially for inserting unions into a struct and retaining API compatibility. It's a shame we can't use them.

[zooba]

I agree they're useful, but I don't need to be convinced. We have to win over the C++ committee 😄 And then wait a decade for it to be generally available in all the compilers we support.

Incidentally, I noticed the SET macro, so I fixed it and am switching the assignments to use that.

[zooba]

#101912 is an alternate PR to this one

[zooba]

We prefer #101912