[clangd] Enable lit internal shell by default
Enable it now that all of the tests pass under the internal shell. The
internal shell is slightly faster (10-15%) and also provides a better
debugging experience.
Reviewers: petrhosek, ilovepi, kadircet, HighCommander4
Reviewed By: ilovepi
Pull Request: https://github.com/llvm/llvm-project/pull/169540
