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

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Sirraide wrote:

> But the template argument list is certainly _morally_ const and it is certainly immutable from the point of view of the caller. I think if we remove all the consts there is a higher chance that we actually end up mutating arguments in a persistent manner and that would be bad.

Yeah, makes sense. Copying does sound like a solution with less room for error if that’s feasible.

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