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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I am repeating myself here, but the crash happens after a bunch of errors: it's not significant, we have evidence this sort of crash is associated with error recovery.

This patch implements a standard mandated breaking change, and this 'stdexec' is user code.
Without evidence that stdexec does something that should be accepted within reason, I don't think that's grounds for revert.

The simplest thing to look for here: Does this project already workaround the changes as they were implemented in GCC?
If so, this could be just a matter of changing a preprocessor test.

Here is what I think would be a good evidence that something is wrong with this patch: A snippet of code, extracted from 'stdexec', that works on GCC, works on official clang, but doesn't after this patch.

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