CI: Make sure libclang is installed on Windows

.github/workflows/main.yml

@@ -95,6 +95,21 @@ jobs:
       with:
         toolchain: ${{ matrix.rust }}
 
+    - name: Install libclang
+      # Note: libclang is pre-installed on the macOS and linux images.
+      if: matrix.os == 'windows-latest'
+      run: |
+        Invoke-WebRequest http://releases.llvm.org/9.0.0/LLVM-9.0.0-win64.exe -OutFile llvm-installer.exe
+        7z x llvm-installer.exe -oC:\llvm-binary
+        Write-Host ::set-env name=LIBCLANG_PATH::C:\llvm-binary\bin\libclang.dll
+        Write-Host ::add-path::C:\llvm-binary\bin
+
+    - name: Query Clang Version
+      if: matrix.os == 'windows-latest'
+      run: |
+        Get-Command clang.exe
+        clang.exe --version
+
     # Install wasm32-wasi target in order to build wasi-common's integration
     # tests
     - run: rustup target add wasm32-wasi