[clang] [clang] [Sema] Implement CXXConstructExpr::getUnusedResultAttr (PR #152950)

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Sirraide wrote:

@halbi2 Just FYI, we usually tag patches that are just refactoring and don’t change behaviour as ‘NFC’ (‘non-functional change’).

https://github.com/llvm/llvm-project/pull/152950