[clang] [Clang] disallow attributes on void parameters (PR #124920)

[Aaron Ballman via cfe-commits]

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@@ -50,8 +50,8 @@ struct testRecoverStrictnessStruct { };
 
 #pragma clang attribute pop
 
-#pragma clang attribute push (__attribute__((abi_tag("a"))), apply_to = any(function, record(unless(is_union)), variable, enum))
-// expected-error at -1 {{attribute 'abi_tag' cannot be applied to 'enum'}}
+#pragma clang attribute push (__attribute__((abi_tag("a"))), apply_to = any(function, record(unless(is_union)), variable, enum)) // expected-warning {{attribute 'abi_tag' cannot be applied to a 'void' parameter}}
+                                                                                                                                 // expected-error at -1 {{attribute 'abi_tag' cannot be applied to 'enum'}}
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AaronBallman wrote:

```suggestion
#pragma clang attribute push (__attribute__((abi_tag("a"))), apply_to = any(function, record(unless(is_union)), variable, enum))
// expected-warning at -1 {{attribute 'abi_tag' cannot be applied to a 'void' parameter}}
// expected-error at -2 {{attribute 'abi_tag' cannot be applied to 'enum'}}
```
I *think* this is actually a bug with the way `clang attribute push` works. A `void` parameter is not a variable because it doesn't really declare a parameter (there's no object backing the parameter like there is for other cases). WDYT @erichkeane?

https://github.com/llvm/llvm-project/pull/124920