[CI] Fix cross-project-tests dependencies (#193323)

We error out in CMake otherwise if we do not enable `clang`. Avoid
adding other dependencies for now as they are optional and otherwise
cause us to build them when using cross-project-tests to test other
projects like lld.

.ci/compute_projects.py

@@ -23,6 +23,7 @@ PROJECT_DEPENDENCIES = {
     "bolt": {"clang", "lld", "llvm"},
     "clang-tools-extra": {"clang", "llvm"},
     "compiler-rt": {"clang", "lld"},
+    "cross-project-tests": {"clang", "lldb", "lld"},
     "libc": {"clang", "lld"},
     "openmp": {"clang", "lld"},
     "flang": {"llvm", "clang"},

.ci/compute_projects_test.py

@@ -450,6 +450,19 @@ class TestComputeProjects(unittest.TestCase):
             "check-cxx check-cxxabi check-unwind",
         )
 
+    def test_cross_project_tests(self):
+        env_variables = compute_projects.get_env_variables(
+            ["cross-project-tests/CMakeLists.txt"], "Linux"
+        )
+        self.assertEqual(
+            env_variables["projects_to_build"],
+            "clang;cross-project-tests;lld;lldb;llvm",
+        )
+        self.assertEqual(env_variables["project_check_targets"], "check-cross-project")
+        self.assertEqual(env_variables["runtimes_to_build"], "")
+        self.assertEqual(env_variables["runtimes_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_check_targets_needs_reconfig"], "")
+
 
 if __name__ == "__main__":
     unittest.main()