[clang] [clang-tools-extra] Remove clang-pseudo (PR #109154)

[kadir çetinkaya via cfe-commits]

kadircet wrote:

thanks a lot for taking care of this @AaronBallman, I think the only concern here is not regressing clangd functionality. We can figure out how we should trim down the bits that're moving into clangd later if need be. I think this LG, but cc @hokein  as well.

https://github.com/llvm/llvm-project/pull/109154