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

[Sam McCall via cfe-commits]

sam-mccall wrote:

All makes sense to me. 

I'd point out that the only revert I was asking for was asking for was of the deprecation of the flag to restore the old behavior, and *optionally* the default flip. 

The combination of {prominent oss library, accepted by clang-18 and gcc, now ICE, flag to fix is deprecated} doesn't seem tenable even if the code is invalid, so I don't think that resolving it hinged on reproducer quality.

Others requested a broader revert. I suspect this is fix-forward being bad for tree stability, and revert + partial-reapply being better in this respect. Which... well, tradeoffs.

In any case I think we lost some nuance here, I'll try to be more helpful in future!

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