[clang] [clang] Improved diagnostics for explicit specialization/instantiation of closure type members (PR #192843)

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StefanPaulet wrote:

> @StefanPaulet I noticed too late that your email is not public. Please make sure that your email is public, as it is required by our policy: https://llvm.org/docs/DeveloperPolicy.html#email-addresses

Sorry, I was not aware of that. Changed it now to public. Thanks for letting me know!

https://github.com/llvm/llvm-project/pull/192843