[clang] [AArch64][Clang] Add support for __arm_agnostic("sme_za_state") (PR #121788)

[Benjamin Maxwell via cfe-commits]

================
@@ -3835,6 +3835,9 @@ def err_sme_unimplemented_za_save_restore : Error<
   "call to a function that shares state other than 'za' from a "
   "function that has live 'za' state requires a spill/fill of ZA, which is not yet "
   "implemented">;
+def err_sme_unimplemented_agnostic_new : Error<
+  "support to handle __arm_agnostic(\"sme_za_state\") together with "
+  "__arm_new(\"za\") or __arm_new(\"zt0\") is not yet implemented">;
----------------
MacDue wrote:

nit: Little shorter/more common phrasing 
```suggestion
  "__arm_agnostic(\"sme_za_state\") is not supported together with "
  "__arm_new(\"za\") or __arm_new(\"zt0\")">;
```

https://github.com/llvm/llvm-project/pull/121788