[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:

I can't claim I understand CWG638 and this entire deal of explicit specializations, but I see that all FIXMEs in CWG638 test have been removed. Should we claim conformance? 

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