[clang] [llvm] [ADT] Fix SmallVector append with input iterators (PR #191030)

[Alexis Engelke via cfe-commits]

aengelke wrote:

FWIW, using the emplace_back append for non-random-access iterators is slightly negative in c-t-t stage1 builds and slightly positive in c-t-t stage2 builds... [c-t-t](https://llvm-compile-time-tracker.com/compare.php?from=a602927e3605d213caadfe3c63a619702b042aae&to=d2caebd062fe1f2cab8323a7946a9f9d3fe0c37e&stat=instructions:u). Not sure whether we should change that.

https://github.com/llvm/llvm-project/pull/191030