[clang] [clang] Add diagnostic for friend declaration of closure type member (PR #191419)

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StefanPaulet wrote:

> Also make sure to run tests, because changes you made in `cwg17xx.cpp` would not pass tests. If you're using `make` or `ninja`, C++ DR tests can be run with `make check-clang-cxx-drs` or `ninja check-clang-cxx-drs`.

The test was passing on my machine. I also used `llvm-lit` to test it specifically. Could you let me know what the error was so I can fix it?

https://github.com/llvm/llvm-project/pull/191419