[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:

Thank you for the reproducers! I have posted a candidate reland that addresses both of these issues here: https://github.com/llvm/llvm-project/pull/73087

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