[clang] [Clang] [Parser] Improve diagnostic for `friend concept` (PR #105121)

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Sirraide wrote:

`template<class> concept friend fooable;` still gives the error ‘expected unqualified-id’; I could move the check for whether it’s a concept declaration at class scope to the start of `ParseConceptDefinition` instead of doing that in `ActOnStartConceptDefinition` or whatever it was called and add a check for whether it’s friend-specified, but I’m not sure that’s really necessary.

https://github.com/llvm/llvm-project/pull/105121