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

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higher-performance wrote:

Huh, that's strange. Any idea why the warning isn't being emitted?

https://github.com/llvm/llvm-project/blob/672eaced6d114656315780140f105e3177742c41/clang/test/SemaCXX/uninitialized.cpp#L1516-L1522

I feel like the check looks correct:

https://github.com/llvm/llvm-project/blob/672eaced6d114656315780140f105e3177742c41/clang/lib/AST/DeclCXX.cpp#L2201-L2211

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