[clang] [clang] deprecate frelaxed-template-template-args, make it on by default (PR #89807)

[via cfe-commits]

https://github.com/cor3ntin commented:

I'm pretty happy with the direction this is going in.
I'd like to see the specific test in https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2398, the ones we exchanged by mails and some tests with constraints

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