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

[Sam McCall via cfe-commits]

sam-mccall wrote:

This commit did three things: A) changed the implementation, B) changed the flag default, and C) deprecated the flag.

Since clang now crashes on widely-used, real-world code, can we at least revert C, and ideally B until the crashes are fixed?

(It would also have been helpful to separate A and B, automated bisection can't tell the difference between the two, and generally we'd deal with crashes in default config by reverting the whole commit)

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