[llvm] [llvm] Use "= default" (NFC) (PR #166088)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Is there some sort of coding standard rule for this change? I'm not convinced personally that `= default` is better than `{}` and I suspect it will be routinely ignored, especially without a coding standard rule to enforce it...

I'm curious about this as well, especially as `{}` is slightly more compact :) 

https://github.com/llvm/llvm-project/pull/166088