[Bazel] Add a comment why an export file is needed. (llvm#115556)

- [x] Add follow-up comment on why the export file is needed in
BUILD.bazel

Author: weiweichen

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -32,6 +32,7 @@ exports_files([
     "include/llvm/IR/Intrinsics.td",
     "include/llvm/Option/OptParser.td",
     "utils/lit/lit.py",
+    # This one is needed for building and vendoring out lldb from off tree.
     "utils/lldbDataFormatters.py",
 ])