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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

I think the main contribution of this patch is showing the extent of the propagation. I personally was surprised this got to `ASTMatchers` and `APValue`. Now that we understand the scope, we can try to draw a line where changes here stop making sense, and do some else where we'd like propagation to stop, e.g. start copying things.

> A clean change would be to copy the whole MTAL and replace the "forgotten" argument.

I'm not sure I understand, because MLTAL doesn't seem to own the arguments in the first place, so copying it doesn't seem of any help.

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