[clang] [Sema] Avoid unnecessary copy on MultiLevelTemplateArgumentList. NFC (PR #84459)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Ah, this used to be a really small class, at one point it was an unsigned plus a handful of array-refs, so it made sense to pass by value.  It seems to have grown a decent bit to the point that pass-by-const-ref now makes sense.

https://github.com/llvm/llvm-project/pull/84459