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

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bgra8 wrote:

> > Here's a preprocessed file: [repro.zip](https://github.com/llvm/llvm-project/files/15250584/repro.zip)
> > I tried to reduce, and got rid of most of the test code and some of the stdexec code, but there's still a lot left. I hit the end of my timebox on that. Maybe creduce can do better.
> 
> That crashes with released clang 18.1.4 as well, same stack trace it seems.
> 
> ```
> Homebrew clang version 18.1.4
> Target: arm64-apple-darwin23.4.0
> ```
> 
> Is it possible you may have reduced into a different bug? It's always helpful in that case to keep testing with known good compiler while reducing.

@mizvekov I have a [reduced test case](https://github.com/llvm/llvm-project/files/15261978/repro.zip) for the crash @sam-mccall reported.

Clang does not crash before but crashes at this revision.

Compilation command:
```
$ clang -std=gnu++20 -fsyntax-only -c repro.cc
```



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