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

[Benjamin Maxwell via cfe-commits]

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@@ -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<
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MacDue wrote:

nit: unimplemented -> unsupported (I don't think this is something we ever intend to implement) 
```suggestion
def err_sme_unsupported_agnostic_new : Error<
```

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