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

[Erich Keane 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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erichkeane wrote:

>Because of the && DestRecordDecl->hasUninitializedExplicitInitFields() conditions a couple of lines below.

Right, but that is a property of the declaration 'elsewhere', not the initialization of the type here.  We actually shouldn't be storing that initialization on the  `RecordDecl` itself, that should be immutable once we see the semicolon.

>Trying to dig up the exact field is complicated and slow.
Complicated? Perhaps somewhat.  `slow` is usually OK for the purposes of diagnostics, it seems to be sensible here, IMO.

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