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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Some of the comments point out to how write `expected` directives in C++ DR tests. You can look at the rest of `cwg17xx.cpp` for examples — C++ DR tests are pretty consistent in this regard, and I'm trying to keep them this way.

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