[CI] Run LLDB tests on Clang changes in pre-merge CI

This attempts #132795 again.
Last time we tried this we didn't have enough infra capacity, so had to
revert. According to recent communication from the Infrastructure Area
Team, we should now have enough capacity to re-enable the LLDB tests.

Author: Michael137

.ci/compute_projects.py

@@ -49,8 +49,7 @@
         "flang",
     },
     "lld": {"bolt", "cross-project-tests"},
-    # TODO(issues/132795): LLDB should be enabled on clang changes.
-    "clang": {"clang-tools-extra", "cross-project-tests"},
+    "clang": {"clang-tools-extra", "cross-project-tests", "lldb"},
     "mlir": {"flang"},
     # Test everything if ci scripts are changed.
     ".ci": {