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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> @mizvekov Thank you! With that patch, clang not only doesn't crash on stdexec with `-frelaxed-template-template-args`, but in fact accepts the code.

Thanks! The crash is still there and is pre-existing, but it's a 'crash-on-invalid' issue, which is lower priority.

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