[clang] [clang-tools-extra] [clang][NFC] Un-constify `MultiLevelTemplateArgumentList` (PR #104687)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I not opposed to exploring a solution now. I suspect there might be a nicer solution that doesn't involve copies, I just have other priorities right now and it will take me a while until I can sit down and think about this.

But if you want to think about the problem and find a solution that doesn't involve a lot of churn, and this is aligned with your priorities, I am not opposed, that could help.

https://github.com/llvm/llvm-project/pull/104687