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

[via cfe-commits]

alexfh wrote:

Can we start with either reverting the whole commit or at least removing the flag deprecation and returning the old default (`-fno-relaxed-template-template-args`)? I think, @bgra8 and @sam-mccall can try to provide a reduced test case without renaming next week (it's sort of a [long weekend](https://www.thelocal.de/20220307/german-word-of-the-day-der-bruckentag) in Germany with some folks not working), but not having a *convenient* reproducer is not a good reason to keep the ToT Clang in a broken state.

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