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

[Erich Keane via cfe-commits]

erichkeane wrote:

>I am repeating myself here, but the crash happens after a bunch of errors: it's not significant, we have evidence this sort of crash is associated with error recovery.

I missed this, thanks for clarifying (godbolt link scrolled and all I saw was warnings!).  A crash-on-invalid is very much not grounds for a revert.

Does someone have reproducer where this ISN'T the case? 

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