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

[Erich Keane via cfe-commits]

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@@ -119,6 +119,15 @@ 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 requiring explicit
+/// initialization with [[clang::requires_explicit_initialization]] in a type
+/// without a user-provided default constructor, or if this is the case for any
+/// base classes and/or member variables whose types are aggregates.
+///
+/// In this case, default-construction is diagnosed, as it would not explicitly
+/// initialize the field.
+FIELD(HasUninitializedExplicitInitFields, 1, NO_MERGE)
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erichkeane wrote:

Hmm... is there a good reason to store this as a bit rather than doing a `hasAttr` check? This increases the cost of `FieldDecl` for everyone.

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