[clang] [Clang] Disallow explicit object parameters in more contexts (PR #89078)

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Sirraide wrote:

Alright, it took me a bit, but I’ve figured out how to continue issuing the ‘... cannot appear in a non-member function’ diagnostic where appropriate. This also handles `friend` declarations now because I had forgotten about that; I’ve also added a few more tests for declarations w/ nested-name-specifiers.

https://github.com/llvm/llvm-project/pull/89078