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

[Kerry McLaughlin via cfe-commits]

================
@@ -7559,6 +7559,32 @@ 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
+affects the function's calling convention for a given state S. This
+attribute allows the user to describe a function that preserves S, without
+requiring the function to share S with its callers and without the making
----------------
kmclaughlin-arm wrote:

```suggestion
requiring the function to share S with its callers and without making
```

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