[clang] [Clang] Add [[clang::complete_on_member_instantiation]] (PR #211231)

[Louis Dionne via cfe-commits]

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@@ -5503,3 +5503,12 @@ def Personality : InheritableAttr {
   let Subjects = SubjectList<[Function]>;
   let Documentation = [PersonalityDocs];
 }
+
+def CompleteOnMemberInst : InheritableAttr {
+  let Spellings = [Clang<"complete_on_member_instantiation">];
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ldionne wrote:

`complete_on_member_instantiation` seems to suggest that the type is completed on member instantiation, or something.

Instead, I would suggest spelling it as `clang::require_complete_on_member_instantiation`. It's longer, but the attribute is already a mouthful, so I don't think that makes it worse.

https://github.com/llvm/llvm-project/pull/211231