[clang] [clang][X86] Ignore ABI-empty fields and base classes in X86_64 SysV ABI classification algorithm (PR #187814)

[John McCall via cfe-commits]

rjmccall wrote:

With that said, a couple notes.

First, the change here is pretty far from generally ignoring empty bases and fields in the classification algorithm; it's ignoring them for very specific checks. We're still recursively classifying and merging information from the base/field type.

Second, I think PlayStation and Darwin probably want to be exceptions to `returnCXXRecordGreaterThan128InMem()` in general, not just for empty classes.

Third, given that, I think this feels more like a bug in the fix in https://github.com/llvm/llvm-project/commit/e8a486ea97895a18e1bba75431d37d9758886084. Maybe we still need to offer an ABI version for it just in case someone's stuck maintaining compatibility with a clang released in that short window, but it's worth considering if we really have to.

https://github.com/llvm/llvm-project/pull/187814