swift-wasm-DEVELOPMENT-SNAPSHOT-2021-11-05-a

Update checkout config to build with upstream LLVM (#3828)

As previously announced by @kateinoigakukun:
>The last required commit for swiftwasm has been cherry-picked to the swift's llvm branch! https://github.com/apple/llvm-project/pull/3451
>We no longer need to maintain our llvm downstream fork