[llvm-branch-commits] [clangd] Enable lit internal shell by default (PR #169540)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/169540

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.