Swift: build with Clang on Linux

.github/workflows/swift-qltest.yml

@@ -32,7 +32,7 @@ jobs:
         if: runner.os == 'Linux'
         run: |
           sudo apt-get update
-          sudo apt-get install -y wget
+          sudo apt-get install -y wget clang
           wget https://github.com/bazelbuild/bazelisk/releases/download/v1.11.0/bazelisk-linux-amd64
           mv bazelisk-linux-amd64 /usr/local/bin/bazel
           chmod +x /usr/local/bin/bazel
@@ -43,6 +43,9 @@ jobs:
       - name: Build Swift extractor
         run: |
           bazel run //swift:create-extractor-pack
+        env:
+          CC: clang
+          CXX: clang++
       - name: Run QL tests
         run: |
           codeql test run --threads=0 --ram 5000 --search-path "${{ github.workspace }}/swift/extractor-pack" --check-databases --check-unused-labels --check-repeated-labels --check-redefined-labels --check-use-before-definition ql/test