[clang] [clang] [NFC] Improve move-assign and move-constructor for NestedNameSpecifierLocBuilder (PR #180484)

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serge-sans-paille wrote:

> Hey @mizvekov I did not try to follow the impact on compile-time-tracker but that wasn't the goal. I originally just wanted to enable a move from #180482. I'll give a try to removing the copy-constructor it totally makes sense for a Builder.

And it turns out copying is needed.

https://github.com/llvm/llvm-project/pull/180484