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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Would it be reasonable to add a `-Wno-deprecated-relaxed-template-template-args` flag (or something like that) for this specific deprecation?
> 
> I had similar idea, but what about instead implementing something generic to ignore deprecation of any driver flag?

I think I prefer pretty fine-grained ones TBH, it makes our deprecation warnings more valuable.  In a perfect world, it would change every release of the compiler so that folks would be frequently reminded of it, but it isn't a perfect world :) 

> Does it sound good for everyone that we revert the deprecation of the negative spelling of the flag for a while, until we come up with a patch for a new flag which helps ignore these deprecations?

Is doing so much of a task? I would expect it to be like other diagnostics and take roughly the same amount of time as reverting the negative-spelling-deprecation.  I don't see the request to remove the deprecation (it IS just a warning afterall!) to be particularly motivating until release time (it IS just a warning afterall!), so I'd think it would a better spending of time to implement what Richard suggested.

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