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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> :( My goal now is to fix xtensor implementation/original tests, so this is not a question of reduction. I need to understand where the compiler picked a different specialization with relaxed argument matching.

So from the reduction you can see you have a problem where `svector` is matching a partial specialization of `xtype_for_shape`, which it shouldn't.

I see that you have a very similar problem to your first one, which is not fixed in main yet, where one of the partial specializations is ifdef'd out in a similar manner, that also very suspiciously looks like it should have been using the feature test macro instead:
https://github.com/xtensor-stack/xtensor/blob/d9c3782ed51027b2d00be3c26288b2f74e4dbe94/include/xtensor/xexpression_traits.hpp#L106

Does changing that to the feature test macro help?

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