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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

> Test failure looks relevant. There were reasons we did not enable the feature test macro in the first place, so it would be good for the patch summary to explain why and what's changed to allow us to enable it today.

Marking as requested changes just to make sure this research gets done before we land anything.

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