[clang] Also run the clang tests as part of the Clang-specific CI pipeline

[ldionne]

Summary

We used to rely on the monorepo-wide premerge checks for running Clang
tests, but I think it makes more sense to run the Clang tests explicitly
in its CI pipeline since we want to move away from monorepo-wide checks.

[ldionne]

Only run check-clang

clang/utils/ci/buildkite-pipeline.yml

- label: "Format"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
   - wait
 
-  - label: "Building clang"
+  - label: "Building and testing clang"
     commands:
       - "clang/utils/ci/run-buildbot build-clang"
     agents:
       queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2

clang/utils/ci/run-buildbot

build-clang)
     mkdir install
     # We use Release here to avoid including debug information. Otherwise, the
     # clang binary is very large, which is problematic because we need to upload
     # the artifacts for other jobs to use. This may seem like nothing, but with
     # the number of jobs we run daily, this can result in thousands of GB of
     # network I/O.
     cmake                                                                      \
         -S llvm                                                                \
-        -B build                                                               \
+        -B ${BUILD_DIR}                                                        \
         -G Ninja                                                               \
         -DCMAKE_CXX_COMPILER_LAUNCHER="ccache"                                 \
         -DCMAKE_BUILD_TYPE=Release                                             \
         -DCMAKE_INSTALL_PREFIX=install                                         \
         -DLLVM_ENABLE_PROJECTS="clang;compiler-rt"                             \
 
-    ninja -C build install-clang install-clang-resource-headers
+    ninja -C ${BUILD_DIR} install-clang install-clang-resource-headers
     ccache -s
     tar -cJvf install.tar.xz install/
     buildkite-agent artifact upload --debug install.tar.xz
+
+    ninja -C ${BUILD_DIR} check-clang
 ;;
 generic-cxx03)
     buildkite-agent artifact download install.tar.xz .
     tar -xvf install.tar.xz
     export CC=$(pwd)/install/bin/clang
     export CXX=$(pwd)/install/bin/clang++
     chmod +x install/bin/clang install/bin/clang++