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

[Benjamin Maxwell via cfe-commits]

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@@ -7559,6 +7559,26 @@ The attributes ``__arm_in(S)``, ``__arm_out(S)``, ``__arm_inout(S)`` and
   }];
 }
 
+def ArmAgnosticDocs : Documentation {
+  let Category = DocCatArmSmeAttributes;
+  let Content = [{
+The ``__arm_agnostic`` keyword applies to prototyped function types and
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MacDue wrote:

Probably just me but "prototyped function types" reads a little oddly, I think I'd normally just say "function prototypes". 

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