Updates to support clingo-cffi (#20657)

* Support clingo when used with cffi

Clingo recently merged in a new Python module option based on cffi.

Compatibility with this module requires a few changes to spack - it does not automatically convert strings/ints/etc to Symbol and clingo.Symbol.string throws on failure.

manually convert str/int to clingo.Symbol types
catch stringify exceptions
add job for clingo-cffi to Spack CI
switch to potassco-vendored wheel for clingo-cffi CI
on_unsat argument when cffi

.github/workflows/linux_unit_tests.yaml

@@ -165,3 +165,54 @@ jobs:
       - uses: codecov/codecov-action@v1
         with:
           flags: unittests,linux,clingo
+  clingo-cffi:
+    # Test for the clingo based solver using the CFFI package
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+      - name: Install System packages
+        run: |
+            sudo apt-get -y update
+            # Needed for unit tests
+            sudo apt-get install -y coreutils gfortran graphviz gnupg2 mercurial
+            sudo apt-get install -y ninja-build patchelf
+            # Needed for kcov
+            sudo apt-get -y install cmake binutils-dev libcurl4-openssl-dev
+            sudo apt-get -y install zlib1g-dev libdw-dev libiberty-dev
+      - name: Install Python packages
+        run: |
+            pip install --upgrade pip six setuptools codecov coverage cffi
+            pip install -i https://test.pypi.org/simple/ clingo-cffi
+      - name: Setup git configuration
+        run: |
+            # Need this for the git tests to succeed.
+            git --version
+            . .github/workflows/setup_git.sh
+      - name: Install kcov for bash script coverage
+        env:
+            KCOV_VERSION: 34
+        run: |
+            KCOV_ROOT=$(mktemp -d)
+            wget --output-document=${KCOV_ROOT}/${KCOV_VERSION}.tar.gz https://github.com/SimonKagstrom/kcov/archive/v${KCOV_VERSION}.tar.gz
+            tar -C ${KCOV_ROOT} -xzvf ${KCOV_ROOT}/${KCOV_VERSION}.tar.gz
+            mkdir -p ${KCOV_ROOT}/build
+            cd ${KCOV_ROOT}/build && cmake -Wno-dev ${KCOV_ROOT}/kcov-${KCOV_VERSION} && cd -
+            make -C ${KCOV_ROOT}/build && sudo  make -C ${KCOV_ROOT}/build install
+      - name: Run unit tests
+        run: |
+          whoami && echo PWD=$PWD && echo HOME=$HOME && echo SPACK_TEST_SOLVER=$SPACK_TEST_SOLVER
+          python -c "import clingo; print(hasattr(clingo.Symbol, '_rep'), clingo.__version__)"
+          . share/spack/setup-env.sh
+          spack compiler find
+          spack solve mpileaks%gcc
+          coverage run $(which spack) unit-test -v
+          coverage combine
+          coverage xml
+      - uses: codecov/codecov-action@v1
+        with:
+          flags: unittests,linux,clingo