[clang] [clang] Replace LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] (NFC) (PR #163914)

[Tim Gymnich via cfe-commits]

tgymnich wrote:

> Looking around there seem to be many more uses of `LLVM_ATTRIBUTE_UNUSED` around - Why replace these ones but not everything?
> 
> (I think doing that replacement makes sense - all the host toolchain have support for the attribute afaik)

I assume this is to keep PR sizes small and easy to review.

https://github.com/llvm/llvm-project/pull/163914