[clang] [clang-tools-extra] [Clang] Reland support friend declarations with a dependent nested-name-specifier (PR #208345)

[Vlad Serebrennikov via cfe-commits]

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Endilll wrote:

We should add a sentence here to the effect of "The 'declaration does not name a member' diagnostic should be replaced with something along the lines of 'name of friend declaration cannot contain a pack indexing expression'.", because otherwise it's not clear why the new diagnostic is not enough.

Separately, how the hell did we manage to claim conformance here? (I know I approved the PR for that, but still.)

https://github.com/llvm/llvm-project/pull/208345