[clang] [clang] Fix `gnu::init_priority` attribute handling for reserved values (PR #121577)

[via cfe-commits]

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@@ -3324,6 +3324,9 @@ def err_attribute_argument_out_of_range : Error<
 def err_init_priority_object_attr : Error<
   "can only use 'init_priority' attribute on file-scope definitions "
   "of objects of class type">;
+def warn_init_priority_reserved : Warning<
+  "requested 'init_priority' %0 is reserved for internal use">,
+  InGroup<InitPriorityReserved>;
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el-ev wrote:

```suggestion
  DiagGroup<"init-priority-reserved">, DefaultError;
```

https://github.com/llvm/llvm-project/pull/121577