[CI] Run clad with clang which has asserts enabled.

This gives us a great deal of coverage. Clang has well-developed assert system
which often can detect malformed AST. This is very essential for clad as until
today I would need to run that by hand, locally on every pull request.

At the moment we rely on the fact that llvmlab has such clang builds but
unfortunately only for OSX.

.github/workflows/ci.yml

@@ -235,6 +235,14 @@ jobs:
             coverage: false
             coverity: false
 
+          - name: macos-clang-runtime9-llvmlab
+            os: macos-latest
+            compiler: clang
+            clang-runtime: '9'
+            coverage: false
+            coverity: false
+            llvmlab_id: clang-r354636-b54157
+
           - name: ubuntu-gcc5-runtime9
             os: ubuntu-18.04
             compiler: gcc-5
@@ -676,6 +684,17 @@ jobs:
           -DCODE_COVERAGE=${CODE_COVERAGE}  \
           -DLLVM_EXTERNAL_LIT="`which lit`" \
           $GITHUB_WORKSPACE
+
+        # Fetch clang with RelWithDebInfo which is useful as it has asserts on.
+        # This allows us to verify if clad produces correct AST.
+        if [ -n ${{ matrix.llvmlab_id }} ]; then
+          git clone https://github.com/llvm-mirror/zorg.git zorg
+          cd zorg/llvmbisect
+          sudo python setup.py install
+          llvmlab fetch clang-stage1-configure-RA ${{ matrix.llvmlab_id }}
+          export CLANG=$(pwd)/${{ matrix.llvmlab_id }}
+        fi
+
         cmake --build . --target check-clad -- -j4
     - name: Build Clad for Coverity Scan
       if: ${{ matrix.coverity == true }}