chore: switch back to clang 8

We observed some rare, random test failures since switching to clang 9, so we are going
back to LLVM 8 for the time being

azure-pipelines.yml

@@ -52,7 +52,7 @@ jobs:
   steps:
   - script: |
       choco install -y msys2
-      C:\tools\msys64\usr\bin\bash -lc "pacman --noconfirm -S make python mingw-w64-x86_64-cmake mingw-w64-x86_64-clang mingw-w64-x86_64-polly git diffutils"
+      C:\tools\msys64\usr\bin\bash -lc "pacman --noconfirm -S make python mingw-w64-x86_64-cmake git diffutils && pacman -U http://repo.msys2.org/mingw/x86_64/mingw-w64-x86_64-llvm-8.0.1-3-any.pkg.tar.xz http://repo.msys2.org/mingw/x86_64/mingw-w64-x86_64-clang-8.0.1-3-any.pkg.tar.xz"
     displayName: Setup
   - script: |
       C:\tools\msys64\usr\bin\bash -l ./script/ci.sh

default.nix

@@ -1,3 +1,3 @@
-{ pkgs ? import ./nix/nixpkgs.nix, llvmPackages ? pkgs.llvmPackages_9 }:
+{ pkgs ? import ./nix/nixpkgs.nix, llvmPackages ? pkgs.llvmPackages_8 }:
 
 pkgs.callPackage ./nix/derivation.nix { inherit llvmPackages; }

shell.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import ./nix/nixpkgs.nix, llvmPackages ?pkgs.llvmPackages_9 } @ args:
+{ pkgs ? import ./nix/nixpkgs.nix, llvmPackages ? pkgs.llvmPackages_8 } @ args:
 
 let
   lean = import ./default.nix args;