[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[via cfe-commits]

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@@ -4561,6 +4569,13 @@ static void TryConstructorInitialization(Sema &S,
 
   CXXConstructorDecl *CtorDecl = cast<CXXConstructorDecl>(Best->Function);
   if (Result != OR_Deleted) {
+    if (!IsListInit && Kind.getKind() == InitializationKind::IK_Default &&
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higher-performance wrote:

> I'm not sure how the `RecordDecl` itself can contain the information as to whether this initialization is initializing everything necessary

It is **default**-initialization that conveys this information. If you have required fields, and you're **default**-initializing, then you're omitting initializers for those fields. Am I missing something?

https://github.com/llvm/llvm-project/pull/102040