[llvm] Prefer const auto& to copying the entire object (PR #99830)

[David Blaikie via llvm-commits]

dwblaikie wrote:

How'd you come up with this change? It'd be good to understand how the change is generated so we know it's updating code in a positive way.

For instance - if the type that `auto` is binding to is small enough, the copy may be suitable (eg: if it's int, or pointer - one or two words, basically) to keep it as a copy (the same as we do for function parameters - int and pointer passed by value, but larger things passed by const ref)

@kazutakahirata as someone who does a lot of this sort of cleanup, they might have a better eye for this sort of change

https://github.com/llvm/llvm-project/pull/99830