[clang] [Clang] Treat `ext_vector_type` as a regular type attribute (PR #130177)

[Erich Keane via cfe-commits]

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@@ -1721,17 +1721,10 @@ def EnableIf : InheritableAttr {
   let Documentation = [EnableIfDocs];
 }
 
-def ExtVectorType : Attr {
-  // This is an OpenCL-related attribute and does not receive a [[]] spelling.
-  let Spellings = [GNU<"ext_vector_type">];
-  // FIXME: This subject list is wrong; this is a type attribute.
-  let Subjects = SubjectList<[TypedefName], ErrorDiag>;
+def ExtVectorType : TypeAttr {
+  let Spellings = [RegularKeyword<"__vector_type">, GNU<"ext_vector_type">];
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erichkeane wrote:

I don't like the keyword here, particuarly when it doesn't match the attribute. For better or worse, these types are a wart from the GNU variants of these, and we should stick it that way.  As a keyword it isn't clear whether this should be treated as a qualifier or specifier, and this patch doesn't answer that at all.

Additionally, the `Clang` spelling was the right one, this is a clang attribute, we should spell it like 'clang.

https://github.com/llvm/llvm-project/pull/130177