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

[via cfe-commits]

cor3ntin wrote:

I don't think there is motivation to revert anything at this point (we should avoid churn), however I do agree this is a regression that we should fix quickly @mizvekov.

(Even if it's not a new bug, it is a newly exposed bug, the pain for users is the same)

If the fix takes a long time to manifest then we should consider more aggressive mitigations.

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