[clang] Reland "[clang][Sema] Use original template pattern when declaring implicit deduction guides for nested template classes" (PR #69676)

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antangelo wrote:

Apologies for the delay, I haven't been able to look into this until now. The patch has been reverted in https://github.com/llvm/llvm-project/commit/801c78d5b474c2319aa8ead44db7ba8cacac4714

https://github.com/llvm/llvm-project/pull/69676