[clang] [Clang] Refactor diagnostics for SME builtins. (PR #78258)

[Matthew Devereau via cfe-commits]

================
@@ -102,3 +102,8 @@ svint8_t missing_za(svint8_t zd, svbool_t pg, uint32_t slice_base) __arm_streami
   // expected-warning at +1 {{builtin call is not valid when calling from a function without active ZA state}}
     return svread_hor_za8_s8_m(zd, pg, 0, slice_base);
 }
+
+__arm_new("za")
+svint8_t new_za(svint8_t zd, svbool_t pg, uint32_t slice_base) __arm_streaming {
+    return svread_hor_za8_s8_m(zd, pg, 0, slice_base);  // no warning
----------------
MDevereau wrote:

nit: every other test in this file does
```c++
// expected-no-warning
return sv...
``` 
instead of 
```c++
return sv... // no warning
``` 

https://github.com/llvm/llvm-project/pull/78258