[clang] [Clang][C++17/20] Add full support to deduction guides (PR #193056)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

Is a release note needed?

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An idea about PR title:

[Clang] Bump `__cpp_deduction_guides` to `201907L` in C++20 mode

https://github.com/llvm/llvm-project/pull/193056