[clang] [clang][CGRecordLayout] Remove dependency on isZeroSize (PR #96422)

[Michael Buch via cfe-commits]

Michael137 wrote:

> (Please move out of "draft" when you think this is ready.)

Updated the PR with some more context. There's still a few instances of `FieldDecl::isZeroSize`/`CXXRecordDecl::isEmpty` around `CodeGen` (see `CGClass`, `CGExprConstant`), but it wasn't obvious to me whether those needed replacing as well. Particularly `ConstStructBuilder::Build` has `[[no_unique_address]]` specific logic.

https://github.com/llvm/llvm-project/pull/96422