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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Oh I see the code already includes workaround for GCC vs non-GCC. It's possible in this case you may replace the workaround with a check for the feature testing macro.

But if this is a new ambiguity not covered by any of the cases I am tracking, it could still be worthwhile to isolate it. It could be we can cover it with new provisional wording.

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