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

[Benjamin Maxwell via cfe-commits]

================
@@ -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
+specifies that the function is agnostic about the given state S and
+returns with state S unchanged if state S exists.
+
+The attribute takes string arguments to instruct the compiler which state
+the function is agnostic about.  The supported states for S are:
+
+* ``"sme_za_state"`` for any state enabled by PSTATE.ZA (including the
+  bit itself)
+
+The attributes ``__arm_agnostic("sme_za_state") cannot be used in conjunction
----------------
MacDue wrote:

Missing: \`\`
```suggestion
The attributes ``__arm_agnostic("sme_za_state")`` cannot be used in conjunction
```

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