[clang] Disable alias template CTAD for C++17 (PR #133597)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

We do tend to offer newer features into older standards when that isn't a burden, as long as we have a warning advertising the incompatibility with the current standard, as we do.

What is the motivation for this change?

https://github.com/llvm/llvm-project/pull/133597