[llvm] [ADT] Add LLVM_UNLIKELY hint to SmallVector::reserve() (PR #178606)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar requested changes to this pull request.

This requires extensive benchmarks to make sure it's actually working as intended across major projects and compilers. You can start with the compile time tracker: https://github.com/nikic/llvm-compile-time-tracker

https://github.com/llvm/llvm-project/pull/178606