[clang] [Clang] Warn on duplicate standard attributes (PR #176252)

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keinflue wrote:

See my comment in https://github.com/llvm/llvm-project/issues/176136#issuecomment-3757651105.

I think if at all this warning should not be enabled by default (either in all modes or C++23 and later, depending on whether this is a DR) as it currently is.

https://github.com/llvm/llvm-project/pull/176252