[CI] Exclude runtimes from being tested as projects

Before this patch, making a change to a runtime directory (like libcxx)
would cause the project to be added to the LLVM_ENABLE_PROJECTS CMake
flag which is illegal as they can only be built as part of
LLVM_ENABLE_RUNTIMES. This patch fixes that behavior. Test added.

Author: boomanaiden154

.ci/compute_projects.py

@@ -112,6 +112,8 @@
     "polly": "check-polly",
 }
 
+RUNTIMES = {"libcxx", "libcxxabi", "libunwind"}
+
 
 def _add_dependencies(projects: Set[str]) -> Set[str]:
     projects_with_dependents = set(projects)
@@ -131,6 +133,8 @@ def _compute_projects_to_test(modified_projects: Set[str], platform: str) -> Set
         # Skip all projects where we cannot run tests.
         if modified_project not in PROJECT_CHECK_TARGETS:
             continue
+        if modified_project in RUNTIMES:
+            continue
         projects_to_test.add(modified_project)
         if modified_project not in DEPENDENTS_TO_TEST:
             continue

.ci/compute_projects_test.py

@@ -161,6 +161,15 @@ def test_top_level_file(self):
         self.assertEqual(env_variables["runtimes_to_build"], "")
         self.assertEqual(env_variables["runtimes_check_targets"], "")
 
+    def test_blah(self):
+        env_variables = compute_projects.get_env_variables(
+            [".ci/compute_projects.py", "libcxx/CMakeLists.txt"], "Linux"
+        )
+        self.assertEqual(env_variables["projects_to_build"], "")
+        self.assertEqual(env_variables["project_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_to_build"], "")
+        self.assertEqual(env_variables["runtimes_check_targets"], "")
+
 
 if __name__ == "__main__":
     unittest.main()