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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Thanks for working on this 

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Can you add a test to test/CXX/drs/cwg17xx.cpp? (and re run `www/make_cxx_dr_status`?

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This change needs a release note.
Please add an entry to `clang/docs/ReleaseNotes.rst` in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

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