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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

For CGExprConstant, the code is checking "empty" in the sense of whether there's a corresponding LLVM field.  So almost certainly needs changes.  Not sure how that isn't causing test failures; maybe there's missing test coverage.

For CGClass, it's not directly tied to the LLVM structure layout, but I'm not sure the generated code would be semantically correct if an "empty" field that isn't isEmpty() overlaps with actual data.

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