[clang] [llvm] [AArch64][SME] Remove SelectionDAG SME ABI lowering (PR #190950)

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

> This looks mostly good to me. Removing "aarch64_zt0_undef" is an IR breaking change so we'll need something in AutoUpgrade. Presumably the attribute is safe to ignore so we can just strip it from any IR inputs?

Why is removing `aarch64_zt0_undef` a breaking change? AFAIK unknown string attributes don't do anything: https://godbolt.org/z/1oxb4csGT

https://github.com/llvm/llvm-project/pull/190950