ci: Install clang on Windows through tarballs

Previously we used the executables built the LLVM project but these
executables are difficult to run in a CI environment, they can
accidentally pollute global state, etc. In testing some of the possible
4-core machine environments for Azure this step would frequently cause
issues.

To assuage these future issues and hopefully make builds slightly more
self-contained, this commit changes to install from a tarball instead.
The tarball isn't provided by LLVM itself, but we use the offical LLVM
installer to extract itself and then we pack up the LLVM installation
directory into the tarball.

.azure-pipelines/steps/install-clang.yml

@@ -26,12 +26,18 @@ steps:
 #
 # Original downloaded here came from
 # http://releases.llvm.org/7.0.0/LLVM-7.0.0-win64.exe
-- script: |
-    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf %TEMP%\LLVM-7.0.0-win64.exe https://rust-lang-ci2.s3.amazonaws.com/rust-ci-mirror/LLVM-7.0.0-win64.exe"
-    set CLANG_DIR=%CD%\citools\clang-rust
-    %TEMP%\LLVM-7.0.0-win64.exe /S /NCRC /D=%CLANG_DIR%
-    set RUST_CONFIGURE_ARGS=%RUST_CONFIGURE_ARGS% --set llvm.clang-cl=%CLANG_DIR%\bin\clang-cl.exe
-    echo ##vso[task.setvariable variable=RUST_CONFIGURE_ARGS]%RUST_CONFIGURE_ARGS%
+# That installer was run through `wine` on Linux and then the resulting
+# installation directory (found in `$HOME/.wine/drive_c/Program Files/LLVM`) was
+# packaged up into a tarball. We've had issues otherwise that the installer will
+# randomly hang, provide not a lot of useful information, pollute global state,
+# etc. In general the tarball is just more confined and easier to deal with when
+# working with various CI environments.
+- bash: |
+    set -e
+    mkdir -p citools
+    cd citools
+    curl -f https://rust-lang-ci2.s3.amazonaws.com/rust-ci-mirror/LLVM-7.0.0-win64.tar.gz | tar xzf -
+    echo "##vso[task.setvariable variable=RUST_CONFIGURE_ARGS]$RUST_CONFIGURE_ARGS --set llvm.clang-cl=`pwd`/clang-rust/bin/clang-cl.exe"
   condition: and(succeeded(), eq(variables['Agent.OS'], 'Windows_NT'), eq(variables['MINGW_URL'],''))
   displayName: Install clang (Windows)