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

[Sam McCall via cfe-commits]

sam-mccall wrote:

I'm sorry that I wasn't able to more usefully reduce the failure cases. When such regressions show up, we usually don't have any meaningful context on the code. For our own code, we have guidelines to try to limit complexity which makes reduction more tractable, but third-party libraries are harder.

For publicly-available code, it's not clear to me how much of the burden should fall on people that identify the problem.
I want to do as much of this work as I can, it's difficult to balance the urgency of providing some reproducer (it gets hard to push for a fix if we wait a week for a good reproducers), the quality of reduction, and other work/deadlines. (As mentioned, the timing was difficult this time as this landed just before a holiday).

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