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

[Erich Keane via cfe-commits]

erichkeane wrote:

I agree with @sam-mccall : I think "crashes on some widely-used real-world code" means we should revert `B` until we can make that case work.  Yes, it is a pre-existing issue, however it means that we aren't "ready" to change the flag default.

However, we DEFINITELY need a useful reproducer, and I'm OK holding off on a revert if we can get a fix in 'the next few days' timeframe.

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