llvm-ar does not handle long file paths correctly on Windows

Currently llvm-ar fails to handle long file paths on Windows. This can result in the failure of various plugin tests. Specifically, I have observed that test-defaulting-plugin and test-defaulting-plugin-fail can both fail. I am marking these as fragile on Windows.

This is tracked upstream as https://github.com/llvm/llvm-project/issues/54559.