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

[via cfe-commits]

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@@ -1419,6 +1419,28 @@ is not specified.
   }];
 }
 
+def ExplicitInitDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+The ``clang::explicit_init`` attribute indicates that the field must be
+initialized explicitly by the caller when the class is constructed.
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higher-performance wrote:

Updated, please take another look.

Per the RFC, we might want to forbid its usage on non-aggregates entirely, but I'm not sure exactly where/how to do that.

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