[clang][bytecode] Use SmallVector for Function::Code (#151821)
This way we can use resize_for_overwrite, which is slightly more
efficient:
https://llvm-compile-time-tracker.com/compare.php?from=7bdab76350970a3ac471da6a30035dd5b7ef14f3&to=b55bd2c74f230e2150e54b0523db6a8426eab54d&stat=instructions:u