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

[Ilya Biryukov via cfe-commits]

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@@ -119,6 +119,14 @@ FIELD(HasInitMethod, 1, NO_MERGE)
 /// within anonymous unions or structs.
 FIELD(HasInClassInitializer, 1, NO_MERGE)
 
+/// Custom attribute that is True if any field is marked as explicit in a type
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ilya-biryukov wrote:

Suggestion: could we mention the attribute name?
E.g. "marked as requiring init with [[<attr>]]"

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