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

[via cfe-commits]

GeorgeKA wrote:

> 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?

It was originally detailed in [125913](https://github.com/llvm/llvm-project/issues/125913#issue-2833762681). The motivation from the reporter was simply that the feature is being allowed in C++17 mode when it's documented as a C++20 feature.

>From your response, sounds like it being allowed is okay.

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