[clang] [clang] Enable C++17 relaxed template template argument matching by default (PR #89807)

[Richard Smith via cfe-commits]

zygoloid wrote:

> The immediate deprecation causes a few issues

On the one hand: waiting to deprecate something that we know we're going to deprecate usually doesn't help anyone. We delay getting the message out to our users, we sometimes forget to do it for the next release, and at best it means it takes an extra release cycle to deliver whatever kind of benefit we were aiming for. So I think that we should generally start issuing deprecation warnings as soon as the new thing is ready and we know that we plan to remove the old thing.

On the other hand: a warning without a dedicated warning flag is, for some of our users, the same thing as an error. And, for some of our users, a deprecation error is the same thing as removing the feature immediately. So that doesn't work in this case :)

Would it be reasonable to add a `-Wno-deprecated-relaxed-template-template-args` flag (or something like that) for this specific deprecation?

https://github.com/llvm/llvm-project/pull/89807