Rollup merge of #74995 - sunfishcode:update-llvm, r=alexcrichton

Update the WASI libc build to LLVM 10.

Among other things, this brings in [the `__main_argc_argv`] patch,
which simplifies the interaction between the compiler and WASI libc's
startup code, which will help work on reactor support.

[the `__main_argc_argv` patch]: https://github.com/llvm/llvm-project/commit/00072c08c75050ae2c835b7bb0e505475dbcd7b9

r? @alexcrichton

diff --cc src/ci/docker/host-x86_64/dist-various-2/build-wasi-toolchain.sh
index e9e4a893476a9f4e12cf012359e8b2acce7b11f7,496639e83db2a4d00c2e5987415b341c59c4ab53..c6db200f8660f36f5e3e36faf4e3692ab83ab551
--- 1/src/ci/docker/host-x86_64/dist-various-2/build-wasi-toolchain.sh
--- 2/src/ci/docker/host-x86_64/dist-various-2/build-wasi-toolchain.sh
+++ b/src/ci/docker/host-x86_64/dist-various-2/build-wasi-toolchain.sh
@@@ -4,15 -4,15 +4,15 @@@
  
  set -ex
  
- # Originally from https://releases.llvm.org/9.0.0/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz
- curl https://ci-mirrors.rust-lang.org/rustc/clang%2Bllvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz | \
+ # Originally from https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/clang+llvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz
+ curl https://ci-mirrors.rust-lang.org/rustc/clang%2Bllvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz | \
    tar xJf -
- export PATH=`pwd`/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04/bin:$PATH
+ export PATH=`pwd`/clang+llvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04/bin:$PATH
  
 -git clone https://github.com/CraneStation/wasi-libc
 +git clone https://github.com/WebAssembly/wasi-libc
  
  cd wasi-libc
 -git reset --hard 9efc2f428358564fe64c374d762d0bfce1d92507
 +git reset --hard 215adc8ac9f91eb055311acc72683fd2eb1ae15a
  make -j$(nproc) INSTALL_DIR=/wasm32-wasi install
  
  cd ..