[llvm] [ADT] Use perfect forwarding in SmallSet::insert() (PR #108590)

[Nikita Popov via llvm-commits]

nikic wrote:

This change also has a small negative impact when building with gcc: https://llvm-compile-time-tracker.com/compare.php?from=9583215d55b639f9fc28400b560f9e66c13db13a&to=2a29d24ba94dac82e838c694595a0a574e505aab&stat=instructions:u

https://github.com/llvm/llvm-project/pull/108590